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@NEws LETI,HB 24Physics, University of York l8 February 1977
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CONTENTSGENERAL
CONTRARY OPINIONS
VIACRYL AND SAWDUST
CONDENSATION
MORE ANALYTICAL RESULTS
BOOK REVIEW
ABSTRACTS
GENERAL
I.1 DEATH OF DR NEV]LI,E WILLIAMS
The untimeLy death of NeviLle Wi]liams on
29th Januany of a heart attack, while he was
visiting the British Institute in EasternAfi:ica, in Nairobi, has nobbed the BnitishCommittee of its new Secretary so soon afterhis predecesson, Derek A}len, had died(N.L.15, Section 8).
Neville Williams had had an earlien heartattack in October 1976, after bis visit toRussia, but as soon as he was out of hosPitalhe thnew himself into his wonk again with hisaccustoned enengyr and this proved too much.
The part which he played at the PublicRecord Office fnom 1950 to 1973 (he was DeputyKeeper fon the last thnee yea:rs) r his skilledauthonship of historical books of the Tudonpeniod and his many othen academic achieve-ments will be neconded in other places. Hene
I shall only say how very sad it is to fosea kind fniend and a wise cor:nselIon. TheBnitish Academy, and hence the BnitishCommittee of the Corpus, will find it hand toreplaee him.
I.2 FUTURE OF THE CV-NEWS LETTER
Section 2.1 contains some adverse nema:rks
on the News Letters by M rTean TanaJ-on. Inorder to clean the air fon a sobe:: discussionI have nesigned as Editor of the News Lettenstto take effect on 27th May when the Corpushas its full meeting in Freibr:rg. It is thuslikely that No.25 may 5e the last CV-News
Letter to appean under my Editonship.
1.3 TOth COLLOQUIUM OF THE CORPUS VITREARUM
The pnovisional pnogramme shows that thiswill be an exceptionally intenesting meeting,with sevenal papens on stained glass of thepeniod between 1150 and 1250, and visits toEsslingen, Lautenbach, Walbung, Str:asbourgand Fneibung.
1.4 CRAFTS ADV]SORY COMMITTEE SYMPOS]UM INYORK
This meeting, held fnorn 9th to flthJanuary, was extrenely successful. It washeavily oven-booked and the list of pantici-pants had to be closed when it r:eached 70 sothat nany applicants wene disappointedtespecially fuom overseas. The programme oflectures and visits stinulated much discussionwhich wiLl be rePorted in the Pnoceedings ofthe meetiag. There wene six discussiongroups which had thein own topics from thefollowing Iist:-
1.4.1 Consenvation: Should we alwaysconsenve medieval glass or allow some todecay; should Post-redieval glass be con-senvedl if so, ane diffenent techniquesrequired; ane speci.al teehniques needed formuseum displaY of stained glass?
1.4.2 Resjonation: How much restonationshould E Eone;--Fow much repainting isallowable; how can loose paint be re-fixed3what ane safe oleaning methods fnom the pointof view of funthen weathering; should pro-tective coatings be used, or exterrialglazings; what nesins are suitable as
adhesives or as fillers; how can nredievalcol-ouns be matched; should copies be madeof important windows; what are the best anti-vandal techniques; what neconds should bekept; how can anyone learn whether, andwhen, particular medieval- windows are beingrestored; how can would-be nestoners gainexpe::ience?
1.4.3 Technical qgestions: Why does nredievaLglass have an unusuaJ- chemical composition;wil-1 modern stained glass windows be dunable tor shoui-d we protect them; why is the paint-work sometimes durable and sometimes not;why does nredieval glass sometimes showsurface -crazing?
l-.4.4 Congratul-ations
Miss Chenny Ann Knott of the CraftsAdvisory Committee must be congratulated onconceiving such a valuable rneeting andl,lrs Vivien Lawson for organising it in suchan excel-l-ent manne]:. Al-so we rnust congrat-ufate l4r David Rymer, of the Institute of
..Advanced Anchitectunal Studies, for havingmade such effective adrninistrative arnange-ments in York to everyonets satisfaction.
1.5 EUROPEAN SC]ENCE FOUNDATION ANNUAL REPORT
Section l-.1 of N.L. 19 contained a b::iefnote about the meeting of a group of stainedglass experts in Munich on 25th Febnuary 1976tunden the aegis of the Euncpean ScienceFoundation. This meeting has now beenreported on pages 24-25 of the ESF Annua1Report for 1976, and I have neceived per-mission to neproduce it in fulI, as foIlows,and to add a short note of mY own.
"1. t2 Pre-s.tr.vg.tlo". .oS medi
The detenioration of the mediaeval glassin the gr:eat cathed::als and churches of Europeis a matter: of incneasing concern. Stainedglass windows alre an important pant of aEunopean cuJ-tunal- heritage and they are al-sohistoric neminder:s of the tradi-tions fnornwhich present-day glass industries have sprung.
Some research work is in pnogress in asmal-l- number of institutions throughoutEunope aiming both at understanding the causesand the mechanisms of the cornosion of theglass and at the etaboration of suitabLepnocedures for cleaning and presenvation.An increased scientific effort and a bettenco-ordination of on-going work is, however,impontant in onden to ensure napid pr:ogness inthe developnent of appnopniate technigues sothat conservation can be effected at a satis-factory rate.
At present art histor:ians and scientistsstudying the different aspects of nrediaevalstained glass have a common fonm in the CorpusVitrearum which fonmally is a commission ofthe Intennational Acadenr-ic Union (IAU), andit was from representatives of the IAU' ofthe Co::pus Vitr:eanum and especially theTechnicaf Connnittee of the Corpus Vi.tleanumthat the pnoposal emerged fon ESF involvementin the fiel-d.
To explone possible actions a smallmeeting with expents was held in Munich inFeb::uany 1976. (The participants in themeeting are listed in Appendix J-). Ttre
"rn"e€tihg" which was chaired by Professor Gentner(Heidelbeng), agneed on the skeleton of arather extensive resea::ch progranme to becar?nied out jointly by several Europeanlaborator:ies. The main items of the progrannne
were:
1. a detailed study of the abi.lity ofdifferent coating ma.i:erials to Protect theg1ass, including the crucial question of r'lhat
happens to the glass unden the coatingl
2. the specification and fabrication of anumber: of rtsimul-atedtt mediaeval glasses to be
used by resea::cher:s all over Europe n thuspnoviding common standards in thein work;3. an account of older ::estoration workdescribing rnethods and r:esults I
4. an increased, bnoadened and mone co-o::din-ated neseanch effor:t aiming at a betterr:nderstanding of the fundarnental mechanisms ofthe cornosion of g1ass.
It tnanspired that the second part of thepnogramme was the easiest to implement. Thespecifieati-on of the glasses was made by theTechnical- Conrnittee of the Conpus Vitrearumand through the good offices of the ESF, theNuffield Foundation in London decided to awar:da gr"ant of €,500 to the Chai.rman of thisCommittee (Pnofessor R. Newton, Sheffield)ofon the actual- fabrication. Fon the othenitems, howevern the ESF has not yet been ableto reach entirely satisfactony sol-utions"Some dnaft neseanch prognarrne which wered::arrn up following the neeting in Munich havehad a centain impact on the on-going wonk butit has not yet been possibl-e to start theenvisaged major reseanch programmes on aco-operative basis.rl
Note by RGN: item 3 does not fully describewhat had been agneed at the rpeting in Munich,because the objective of the study of ear:fiennestorations was to identify any damagevisible today which could have been attributableto faulty conservation procedunes many decadesago. Thus it was hoped that the study wouldhelp to pnovide a much-needed time scale fonthe r:ate of development of damage to glass.Thene are likeIy to be so few examples whenethis could be done that it is essential toconduct the study on a European scale.
The Secr:etaniat of the ESF was representedat the CAC Symposium in York (Section 1.4above) and it is clear. that there is sti1lgreat interest in the idea of trying to launcha joint European reseanch prognamme in thisfie1d.
.]-.6 CORRIGENDUM - CLEANING GLASS WITH CALGON
AND EDTA
I am gr.atefuf to Dr.J.C. FenrazzLrLL forpointing out that N.L.23 contains a misunden-standing about his researches on the effeetsof Calgon and EDTA on nedieval g1ass.
Dr Fer::azzini pointed out that thesechelating agents, which can convert insotub.l-eweather:ing products such as gypsum andsyngenite into soluble compounds, cannotnecessarily distinguish between the calciumin the r.eathering cnust and the calcium whichforms an essential- part of the unaltered glass.Hence the glass can be attacked by theseagents. This is confirmed by the work ofEnnsbenger abstracted as No.254 in this NewsLetter becarrse he used EDTA to etch modernglass .
In N.L.23 f stated that Figs. 1 and 2
showed how medievaJ- g.l-ass was damaged by EDTAEffiGlgon, respectivelyo but this is mis-leading because such vis.ible attack occuns onlyon fre€hbljFlgglul'gd slnfqces. But thene isalso anothen fonrn of attack which is invisiblet o--Ihs--qg! e q-SJg. !,le a th e ne d *e d i e rra ]@s s-r,frrich has been eleaned with Calgon or EDTA hasa sunface which apparently has a reduced con-tent of metai ions and is about twice assusceptible to cornosion as is glass which hasbeen cleaned by mechanical- methods. (RGN -
. we do not yet know how thick is the surface'layer. which has been affected by EDTA. If itis ver:y thin, this doubled susceptibility tofurthen attack may not pe:rsist fon too long aper:iod of time.) To repeat: the invisibledamage which has been caused by the EDTA isrendered visible when the glass is subsequentlyexposed to furthen weatheningo and Fig.3 ofN.L.23 shows the damage which occuns afterfr.rthe::. w.ea.thering of the cl-eaned surffi
Thus it was wrong of me to suggest (onpage L2 of N.L.23) that ther:e was a confl-ictbetween Dr Fer:razzini (on the one hand) andDn Bauen and M Bettemboung (on the othenhand). AJ-l three agnee that glass which hasbeen fr"eshl-y-cleaned with EDTA shows no sigrrsof attack which ane visibl-e to the naked eye,but Dn Fen:azzini for:nd that subsequentweathering reveals the danrage. Dr Baue:: andM Bettemboung did not continue thein nesea::chesto discover what happened to the cleaned glasswhen it was again exposed to weathering, butDr Bauer: did envisage the need to make fu::therstudies of the nature of the cleaned surface(see p.64 of Verres et R6fractaires 1976, 30).
I.7 PROFESSOR L.L. HENCH
Pnofesson L.L. Hencho of the Departmentof Mate::ials Science and Engineering,rJniversity of Florida, has been doing muchwork on the durability of glass which is ofinte::est to scientific consenvators, seeo forexample, abstnact 232 in N.L.2l- and abstracts238,239 and 243 i-n N.L.22; two more anticJ-esof his wif.L be abstnacted in N.L.25. In ar:ecent letter to me he says:
.rrl am cumentl-y working veny hand with mystudents on completing the wr:iting of a ser:iesof investigations on the weathe::ing of glassusing the instrumental tooLs of infna-r:edref.l-ection spectr.oscopy o Auger electlronspectroscopy and e.lectnon mic::oprobe. I thinkthene is considerabLe hgpe f::om our studies tobe able to develop the infra-red reflectiontechnique as a means of non-destructive
evaluation of the extent of detenioration ofhistorica.l- windows and identification of theplrocesses needed to restone the windows. We
a::e also completing an investi-gation (Mr Mor::is.Dtlfiorers PhD disser.tation) which may give someclues as to how to appl-y a sunface preserv-ative to the glass without destruction ofoptical pr:openties. 1,1:r Dil-noners work is nowin the final draft stage and I should be ableto send you copies of those papers within thenext three months.rl
I.B SIMULATED MEDIEVAL GLASSES
In Section J-.6 of N.L.20 it was statedthat the Nuffield Foundation had awarded agrant which would enable ten new syntheticmedieval- glasses to be made, fon use inexpe:rimental work on conservation. Theseglasses have now been nede by PilkingtonBnothens Ltdo and all have been carefullyanalysed so that they can also be used foncal-ib::ation of analytical equipment.
Ful1 details of the analyses wil-f begiven in N.L.25 and l-inr-ited quantities will beavailable to genuine expeninenters. Will-anyone interested please ::egisten th,eirinterest with me, stating:-(a) the purpose foir which they(b) the fonm in thich they want
pieces, eg l5mm disc, 50mm
20x5Omm rectang.l-es, etc,(c) the minimum numben of test
required.
need the glass,tbeir test
qJS(j,
My addr:ess is:- Pnofessor R.G. Newton,5 Hardwick Cnescent, SHEFFIELDT S11 BWB,
England.
1.9 MORE 12th CENTURY BLUE SODA GLASS
The rather unusual highly-dunable 12thcentury bLue soda glass which has been foundat Chantnes Cathedral, at the Abbey Chunch atSt Denis, at LrAube, and at York Minster C72pieces only) o would be expected to occur alsoat other places which have .l2th centr.rry gl-ass.Somewhat simila:: gl-ass (but not blue incoloun) has been found at Phrygia, in Tunkey.Searches for this glass have been nade atCantenbuny Cathednal but so far none has beenfound. Howeven, quite unexpectedly, some hasbeen found at two places in the south ofEngland.
I am indebted to Suzanne Keene, of theWinchester Research lJnit, foi: dnawing myattentj-on to some highly dunabLe glass whichhad the same lovely bl-ue colou:: as that atYonk. It came frorn the excavations of the ol-dMinster thene, and I am gr:ateful to M:: MartinBidd.l-e, MA, FSA, Dinecton of the Unit forallowing me to make tests on two of thesamples (Nos. 514 and 515).
SimiJ-arlyo I am indebted to Mrs EleanoreSaundens, of the Salisbury and South WiltshireMuseum, fo:: showing me similar: glass from theexcavations of Ol-d Sar:um, and I am grateful torvlr Peter: Saundens, BA, AMA, Cunator of theMuseum, fon making two sanpl-es (Nos. 516 and517) available for tests.
Analyses of these new samples were kindlycarried out on the XRF equipment at YcrkUniversity by Mn Mank Polland, and I arn grate-ful- to Dr G.A. Cox fon permission to use theseresults. We now have analyses of nine 12thcentuny high-soda glasses and the seven most-i"ntenesting constituents a:re listed in thetable below. Afl- ane blue glasses except thetwo from Phnygia, No.97 being green and No'98yellow.
The two samples fnom Winchester travenemarkably sinilir cou4rositions but they differfron all lhe other:s in containing surprisinglylittle potash, and having by far the highestsoda contents. The two samples from 01d Sarum
differ sufficientJ-y to suggest that they were
not made at the same tine. The sample fnom
LrAube diffens fnom all the otlrer Europeanones in its linre content and the virtualabsence of lead.
At the moment all we can say is that thefour new glasses alre very interesting in beingnot dissimilan f::om the others from England ornonthenn Fnance (the analyses of the glassesfnom Chartnes and St Denis have not yet been
neported in fufl) and alf consenvatorstanchaeologists r museum curlators and arthistonians should dnaw my attention to anymone glass of this kind. In the meantime we
await the results of the Neutnon ActivationAnalysis being cari:ied out on the four new
glasses at the Univensity of Bradford.
Tabl-e of analvses of l2th centurv high-soda glasses
Origin of glass
Analysis No.
Sample No.
York LrAube Yo:rk Phrygia Winchester
515
01d Sarum
5-Lb 5r /2
228
trn
6
93
2
9B
514s7
sJ--L1Ca
Soda
PotasnMagnesia
Lime
Copper oxideLead oxide
61.7
L2,+6.4
8.50 .15
0.31
67-014..3
ZtO
9.0
70 .5
f7" 6'
0"6
014
6.8n ao
0,46
68.7 68,2
r3.7 r2,81.3 r,20.07 r.8
T2,L 7,9
0.39 0,220.005 0.46
70.012"8L.2aa
70,7
16..7
0"4
0.310.61
ConsequentlY, the F::ench TechnicalCommittee has decided not to nepty in the News
Letten to the attacks made on it. In futuresEe-cornmittee will only publish its nesultsin an. international journal of a r:niversallyacknowledged scientific chanactet'.
2.2 EXTERNALLY-VENTILATED 10mm CAVITY
The ttkite-flyingrt exercise in Section 2
of N.L.22 has attracted sonp valuable attention.Two anchitects have wnitten to point out thatthe use of zinc in conjrmction with copper inFig.2 is unsound on el-ectrochemical grounds.
Mr Martin Caroe ARIBA has kindly written:ItIf it is necesany fon the material- to haverigidity in onder to pnovide adequate strengthto the top of the panel, I would suggest anattempt to obtain similarly perforated coPpersheet of penhaps 22'24 gauge. If rigidity isnot important, why not use ondinary copper f1ywire nor:malJ-y fixed in larden vents? .... Do
not use the very fine meshes .... ds I havefound these comPfetely clogged after 20 years t
set:vice . rl .
Dr Bernard Feilden, CBE, FSA' the anchi-tect to Yonk Minsteno has made some modifi-cations to the proposal so that it can be
tested in a window of the wonkshop. His mainmodifications are to intnoduce a spacing
70.6 66 .6
r4.9 13.9anf,i / ol /
n ( l -Lr
7.9 9.1o,24 0.260.61 O,'74
2 CONTRARY OPINIONS
For the finst tinre we have three rrcontnany
opinionstr,
2.I FRENCH TECHNICAL COMM]TTEE
M Jean Taralonr InsPector Genenal ofHistoric Monuments in Fnance and Chairman ofthe French Technical Comrnittee of the Corpus thas asked rne to insert the following note inthis issue of the News Letten. I have alsoreproduced his letten and note in facsimileform so that there sha1l be no doubt aboutwhat he saYs.
Tnapsl"a3ion
The last issue of the News Lettefincluded PaPens vrlrich wene rnore polemic thanscientific in cha::acten, being aimed solelyat attemptinSo bY every means, to discreditin the opinion of the public the reseanch andthe work carried out in France on the conselrv-ation of stained glass windows, and in thisway obstructing the policy which oun Ministeris pursuing in this field.
The French Technical Comrnittee of theCorpus Vitrearum considers such attacks to be
conirary to the objectivity which should be
obsenved by a bulletin beaning the seal ofthe Corpus Vitreanum (CV), whose authonity itis thus rnisusing fon ends incompatible withits true purpose.
block of wood, lOrnm wide, at the bottom ofthe window and a vertical 1ip on the T*ban tostabilise the outen Panel. The ventilationslot at the toP of the window wi1l have a leadshield to exclude dniven rain.
The expeniments will also invoLve the use
of diffenent tneatrents of the externalglazing to improve its appeanance, both fnomoutside and inside the building. Thus the
"kite-flyingil has amply justified its PunPosein getting PeoPIe to think about ways ofpnoducing neally nanrow inte:rspaces '
2.3 DUMB]L]TIES OF SODA- AND POTASII-GLASSES
This rrcontrary opiniontr is by ue againstmyself! It is well known that in simple.(lrtari-siricate) glassesr those containingpotash (X20) ane much less durable tlran the.ooo""ponding soda (NazO) glasses, by a factonof fO or more, depending on the attackingagent used (see, for example, Douglas, R'!1"
*d nf-Sft.tnyl T.M., J.Amer. Ceram' Soc', 1967
50 p3 and Hlnch, L.L.' J. Crystalline Solids,l-975 19 p32) .
I*urr"o, in cornpfex glasses wh'ene.
appreciable amounts of time and magnesra are.i'"o p"."uttt, in addition to potash.and silicatirte al""*iriiy is gneatly improved Csee
El-Shamyo T.M., Phys. Chem. Gl-assest 1973 14
1-5). ih" 1i*u and magnesia stabilise the
3 VIACRYL AND SAWDUST
In N.L. No.20n Section 6.2 pages B-9r Igave an account of the first pant of an
experiment intended to learn whether Viacrylnesin might Pnotect poonJ.y durable glass fnomdetenionation in a particulan accelenatedweather:iag test. Accelenated weathening testsa:re notoriously difficult to devisen and eachtest proPosed has been criticised in sonre wayor anothen because some featu::e (e.g. theIiquid on the ter@eratune) must be made more
aggnessive than in normal weathening. The
aggoessir.e agent used in this acceJ-erated testwas wet English oak sawdust and the tempen-ature was lowen than nonmall it is by noreans an ideaf test but it has the advantageof produeing a definite effect on unpnotectedglass in a period of about half a yeantper.haps without being excessively aggressive.
To remind readers of the finst phase ofthe experiment, Part of the Canterbury GlassRestoration Studio research Progranme was
concerned with the protective capacity ofViacryl. In this test Mr Derek White air-bnaded a sample of Glass No.2 fon 30 secondsusing No.3 powden. Ihe sample was then sentto Sheffiel-d so that the surface noughnesscouLd be neasured with the Talysurf instru-ment. In orden to be able to locate theexact region where the Talysu::f neasunementswere to be made r scllatehes were made on theglass: cr3osses at A pnd B (in Fig.f) andlinean marks at C and D. The rrcentral-rl
measurements (Iine l) wene nrade between thebar of the cnoss at A and the line at C, and
netwonk and the diffenence in durabilitybetvreen a complex soda glass and the corres-ponding potash glass will be gneatly neducedtso much so that I have angued that the differ:-
:6nie wifr be negligible even tbough El-Shamydid not study cornplex soda g1a.sses. It notlseenrs that I was wrong.
I have necently been examining theanalyses of about 200 glasses fnom Yonk
Minsten, cannied out by Mank Pollard at thePhysics DePartment of the Univensi'ty of York"Ii r.rnr"eathened medieval glasses are consider"ed,."a-mmE'="da grasses ane ignored, there isan indication (not yet a proof) that thegraffim contain soda and potash are more
Iunable than those which contain mainlypotash and littl-e soda.
Hene I should fonestal'I those pensons
who wiI-l ooint to the ttrnixed alkali effectrlby r:ennrking that my studi'es wene b-ased on
r6f." "o*poiitiot", and that L.L' Hench
tTElA, p3-) showea that there is no evidenceof a imixed alkali effectrt on the durabilityof glassn provided the compositions arecomiared-on a ry.$g-!gp.. The,rrwe1l-knownrlmixed atkali effect has been a fa-Lse con-clusion based on the comparison of glasscompositions on a weight-percentage basisowheie the molan pnoportion of silica had been
' inadver"tently changed when the alkali had
been changed from soda to Potash'
tj:e other two measurenents (lines 2 and 3)
wene rnde as shown in Fig.l.
After these first measurernents were rnade
(in Septemben 1975) the sample was returnedto Cariterbury to be coated with Viacryl and
then sent to Yor:k to be p'Laced in the wet t'
sawdust for 4] months. The coating was thenremoved with cital I2-L2 (it was easilyremoved) and the Talysurf measurementsrepeated (in Febr:uary l-976). It was fol:ndthat the roughness of the surface haddecreased (see Tabl-es I arrd II on p.8 of N'L'No.20)n suggesting that (as in the expe:rimentwithoui Viacr:yl reported jn Section 6.1- ofN.L. No.20) the slightly rracidicrr liquid fromthe wet sawdust could have etched sore of thesurface away.
ft was pointed out that, in that expeni-nent, the Viaenyl had been applied in one coatonly'and thus it was poss5-ble that ilpin holesrlmigirt have been present in the coating and
nignt nave atlowed the liquid to pass throughand reach the glass. Ttre same piece of glasswas therefone returned to Canterbury in orderto have two successive coatings applied, thefinst being allowed to harden for 3 days
before the second coat was applied. It was
then neturned to the wet sawdust for 1O rnonths(3t6 days). Unfortr:nat61y, as will be seen
below, the sample had not been leaded and
.hence the borden was not protected below theViacnyl coating.
tB
-"----3------1-----2
D
I
Fig. 1 This shows the positions of the original reference marks, scribed on the airbraded surface of the glass
in September 1975. Lines 1,2 and 3 show where the three 'Talysurf' measurements were made on each of the
three occasions.
Fig.2 rhis shows the many blisters which had developed in the viacryl coating after exposure to wet sawdust
foi 10 months (3,16 days). Each blister has a white area below it, and the group of four blisters at (1 ) have one
large white area beneath them all. When the hole was made in the resin at (2), the white area became larger but
no liquid came out and it is assumed that there was only air beneath the blister.
Fig. 3 After removal of the Viacryl coating, the cross at A could easily be seen but the cross at B and the lines
at C and D were;lifficult to locate, suggesting that the surface had again been etched by liquid from thesawdust which had penetrated the Viacryl. lt is possible that the cross at A had not been attached as much as
the other marks because it had been under the large blister at (21 in Fis.2.
When the sanrple was removed fnom tlre saw-
dust, on 6th January 1977r it was found thatthe su::face of the Viacryl was no longensrnooth, but was covened in raised blisterswhere ihe adhesion between the glass and theViacryl had been impained by this exposuxe towet sawdust. Glass No.2 is very dar:k incoloun but thene was a white area under eachb]-isten (see fon exanple area 1 in Fig.2)'\When a bl-isten was pilnced (as at 2 in Flg'2)the white anea incneased in size but no
liquid care out and it seems that thene was
aii beneath it. It is not known how the aircame to be there. The sanple was shown, inthis condition, to the members of the StainedGlass Symposium in York on 9th January'
M:r Peten Gibson then kindly ::eupved theViacnyl coating with Green Label Nitnomors(it was again easily removed) and the members
of the to-tit g could see that the airb:radedsunface did n6t show any obvi.ous visible signsof danege, in panticulan thene were no marks
on the su::face eonnesponding to whene theblisters had been.
the sample was then taken to Sheffieldto be measured on the Talysurf, and it was
only then nealised that thr:ee of tbe indexmanks had nearly disappeaned. The cross atA could still be seen (see Fig.3) but themarks at B, C and D were extrenely difficultto locate, suggesting that the surface had
again been etched by the tiquid, presurnablybecause it had penetrated the Viacnyl becauseit was unlikely that pinholes in eacb of thecoatings would have been opposite to each
other.The Talysunf rrCentre Line Avenagerr
figur:es in micrometres were:-
SePt 1975 Feb 1976 Jan 1977
Line 1 3.1Line 2 2.5Line 3 2.8
2.O 2.72.L 2 .52.5 2.5
and they showed that the surface had become
slightly rougher but it is believed that
4 CONDENSATION4.I MEASUREMENT OF CONDENSATION - PART II
In Section t.B of N.L.22 doubts wereexpressed as to whether the eondensation gauge
described therein rnight not rrdry outil as
quickly as the window does after condensationhas occunnedl in othen words the r€sPonsefrom the condensation gauge might be too sloweornpared with the disappearance of condens-ation from the window. Mr Stephen Moehn, ofthe Physics Departnent of the University ofYonk, has now succeeded in making a very thincondensation 8a1€er vhich is onty O.Bmm thickand has a r€sPonse tire of less than l-O
minutes. When no condensation was Present on
the window the resistance was of the onden of5OOTOOO ohrns. As soon'as light condensationwas visible the resistance had fallen to46rOO0 ohms, and as condensation increased
appneciable etching had occunred, because thescnatches on the sunface had neanly beenoblitenated. If a tead bonder had beenapplied in Febr:uany 1976 there would have been
" -the*possibility of having had a protected edgeand of nreasuring the depth to which the faceof the glass had been etched below it.
This expeniment is oPen to challenge, ifonly on the gnorlnds that wet sawdust is not atypical weathening agent (but it might be nor.ionse than pi.geon dungl) and that the surfacebecame rougher during this exposure for 10months wheneas it becane smoothen du:ring thepnevious 4] rnonths. However, the presence ofihe blist"rs in the surface of the resinn and
the loss of visibility of marks B, C and Dt
suggest that something unexpected had happenedboth to the coating itself and to the glassbeneath the resin coating.
The sample was again sent to Canterbunyv*rene },tn Cole extended and deepened thescnibed rnankings using a dianond point. He
. then leaded the glass all aror:nd the edges
and cemented the leads, using a wooden tool inorden to avoid darnaging the refenence surface 'Ihe whole speeiren was then thoroughly de-greased with acetone and the resin coating(8Oeo ViacnyL and 2oeo Desrnodun w,/w) appliedwith a bnush to cover both the lead and theglass, on the prepared surface only. Aftendrying fon 24 hours at 2J-oC a second coat of::esin was applied but on this occasion themixture was dituted with ethyl acetate (tornake it more fluid) because the ori-ginalmixtune was very viscous and entrapped rnany
air bubbles which still remained aften it hadd::ie d.
Mn Cole also nenranks that two l-2thcentuny bonden panels were coated with Viacrylin May 1975 and placed in the cler:estory atCaaterbuny Cathednal. These were removed forinspection ea::1y in January ]977 and closeexamination showed a wide dist:ibution ofsrna]-I cracks surnounded with a whitish areawhere the rnembnane had detached from the g1ass.
bver the next 16 minutes the resistance fe]-lto IO'OOO ohms. A ventj.Iation fan was thenswitched on and vi-si-ble condensation hadclisappeaned in B rn:inutesr the ::esistance thenUeing ZOT0OO ohms. The fan was kept on and,in the next 1O rrinutes, the resistance hadincreased'to 4OOTOOO ohms. This speed ofresponse is negarded as satj-sfactory.
Also, in the Physics Department, Mn J'S'Scott has devised an electnot'ric counter: whichcan be adjusted to start rd:en condensation ispnesent and stop when it clisappeans. This isnow being used to test the perfonrnance of thenew gauge unden a variety of conditions.Details of these tests can be found in theYork Glaziers Trust Reseanch Report yGT/77 /I,and copies ane ava'ilabfe f::om Mr Peter Gibsont3 Deangaten Yonk Y01 2JrI, at a cost of 75p'
rCONDENSATION IN FREIBURG MINSTER
MUnsterbaumeister Klaus Geis has pre-par.ed a report on the interior cfimate andcondensation in Freibung Minster, datedOctober 1976. The title is rrRaumkl-ina r:nd
Schwitzwasse:rbildung im Freiburger Miinste::rland I have pleasune in summanising it hene.
Ttre document starts with a discussion ofthe causes of deter:ioration of medieval windowsand it goes on to state that since 1970 tberehas been a lo-year programne to fit externalprotective glazing to the Minsten windows incoll-abonation with the State, the Diocesenand the Town.
The thick panes of glass are fitted inthe former glazing grooves of the stainedglass windows I the stained gl-ass panels aremoved inwards about 50mrn and fixed by rneans
of meta} lugs; an airspace of 5 to 10mm isleft-open between the masonr:y and the brassframe of the stained glass panels in o::der toventi.l-ate tbe inter:space. Since eondensationalso has a harmful- effect on stained g.l-ass
'windows it was impontant to establish whethercondensation occuus on the windows unde:: theconditions pnevailing in Freibur-g Minster.From March 1974 to Septemben 1975 the cl-imatein the main aisle was automatically recondedand the micnoc.l-imatic conditions between theexternal pnotective glazing and the stainedglass panels were investigated.
Condensation: The rel-ative humidity in affiffi.termined by its ai:r temperature.Given a water vapour content of-e.g. Bg waterp"" *3 (absolute humidity e g/m3)n then at anlir temperatune of t7o the relative humiditywill be 55eo; if the air tempenature thenfalts^to 7.9o, the absolute humidity nemainsB g/mr but the r:el-ative humidity rises toLOOeoi this is the dew point; condensationfonms on all the col-der" sur:faces in the room.Fnom the neconds of the ::elative atmosphenichumidity and air temperature in FreiburgMinster it is possibl-e to calcul-ate theabsolute humidity; if there is a sharp fallin the air temperatr.me while the absolutehumidity rennins constant a relative atmos-pheric humidity of up to lggeo (the dew point)can be reached. Measurernents of the ne]atlveatmospheric humidity and the air temPeraturein the Minsten, and subsequent measurenent ofthe tempenature of the inside surfaces of thewr'ndows, make it possible to assess whetherthe dew point is neached and consequently therisk of condensation occurring.
l-st senies of measurements: The air: temper-ffiwererecordedcontinuously at two points inside the Minsterfor 1l years: one measuning point was in thesouth aisle near the Schusten window" 4m
above the floorl the other was l8m high inthe nave. Thene was an unexpectedly closeagreement between the ain temperatures andrelative humidities measuned at these twopoints; in the main, the difference did notexceed 1o; the cinculation fi:om the newlyinstalled hot air heating functioned satis-factor:iIy up as fan as the 27m high roof ofthe nave; the Minster heating comes on
between 12o and l3oC. These uniform spacetempenatures occur not onl-y when the hreatingi-s on, because thene is no temperaturestratification in the sumnen eithen.
Mea,suned intenion ternperatr:res :
Mean vafues for: the rneasuringperiods 1974/1975
Maximum monthly mean(in August 1975)
Minirmrm monthly nean(in October f974)
Maximum hourly mean(in August l-975)
Mininun hourly mean during theheating period
(for 90% of the time the measured vafues we::ebetween l-3o and 21o)
Rel-ativs- atmospheric hunriditv uEasur:.e.d in theinterior:
16.9/l-7 .4o
na r OZ1.J
r3.60
cl ao
12.oa
54 /55e"
65 .7 eo
^- 1O-
J I . L'o
80.0%
28.O%
7 .s g/n3
12.2 g/n3
4'9 g/*3
Mean values for the measuringperiods 1974/1975
Maximum monthly nrean( in Septernben 19 75 )
Minimum monthly rnean(in Febnua::y 1975)
Maximum hourly mean(in June 1975, a rainy peniod)
Minimum hourly mean(in March l-975)
Mean values fo:: the measuringperiods L974/1975
Maximum monthfy illean(in September 1975)
Minimum monthly mean(in Febnuary l-975)
(fon B59o of the time the measured values we::ebetween 439o artd 679o)
Absol-ute huniditv (calculated) in the inter:ior:
2nd series of meesurernents: From September:1974 to September 1975 the temperatures werenecorded continuously on two inside windowfaces: 2 x 2 temperature gauges were instal.l-edat the sane height on the Schuster window - onthe inside of the outer window (face No.2) andon the back of the painted glass window (faceNo.3) - in order to determine the surfacetemperatures of the glass in the spaee betweenexterna.l- protective glazing and innen gl-azing.0n the basis of the climatic data fon theinterion of the buildi.ng the dew point wascal-culated for the col-dest glass tempenatunemeasured in the course of a day.
Even the tempenatune on the outer windownever neached the dew point;, no condensationat al-l was observed during the measuringper"iod. In general, it can be said that tLreteTnperature of the outer: glazing sLowl-y fol.l-owsthe tempenature outside the chunch, while thetemperatupe of the stained glass window - theSchuster: window - is always c.l-ose to that ofthe interior of the church. In winten thedanger of condensation is kept at bay by theheating inside the church; but the temperature
shoutrd not fall below that of the 12o - 13oCmaintained so far. No additional- electnicheating of the space between the inner" andouter windows is then needed to preventcondensation.
The rneasurenents also showed that panesof different colouns are heated differently bythe sr:n - there are warm and cold col-ours.The space temperature folJ-ows the extennaltemperature relatively slowln especially inview of the fact that at Freibung the windowaneas are not large companed with the heavymasses of masonryl thene is little aireirculation through the entnance doors andthe instal-lation of the extennal pnotectiveglazing without any leads neduees the heattransfer through the windows. Today, thel-eads of the inner stained glass windows actas heating bridges between the noom tempen-ature inside the church and the tempenatureof the interspace; befone the installationof the extennal protective glazing they wenecooling bnidges, and that is why thene waspreviously a greatelr danger of condensationfonming on the stained glass.
The frequent cLimatic variations, typicatof Fneiburgo have only a gradual effect onthe interiorg even the atmosphenic moistur:einside changes only slowly and it usuallyremains in the medium range. Similar"lyr theart treasunes - wooden earvings, oiJ. paintings tstained glass windows - exp€rnd and contnacts1ow1y. The fairty constant interior climateis a contributory factor in the good conditionof the interion of the building - stonework,deco::ative work. It rtay be assunred that evenbefore the installation of the Minsten heating,which is backgnound heating to a npderatetemperature, the intenior temperature wasfairly even. Ttre cJ-irnate i-n the chancel- offbeiburg Minster, with its lange areas ofglass, which is possi-bJ-y quite diffenent, hasnot yet been investigated. Investigationsinto interior climates at Augsburg andNur:emberg are being canried out by Dr: Frenzel.
These investigations were rnade possiblethr"ough the genenosity of Dr G. RCn:ricke andDipl.Ing.arch. R. Rdnicke, who worked withoutany fee to a total- cost of 8OO0 D.M. andcar::ied out the ineasurements themselves. Ttre
5 MORE ANALYTICAL RESULTS
Many nrone anal-yses of nredieval glassesare now being camied out, i.n severaL diffe::-ent labonatories. Moreover, a few glasseshave been analysed in mone than one labonatoryand some disturbing differences have beenfound. This section of the Ness Letterdiscusses these diffenences (5.1) and alsogives the nesults for 25 mor"e analyses (5.2).
5.1 ACCUMCY OF TTIE F]GURES
Chemists ane accuq,tonred to the idea thatthene ane ernors in thein results, despitethe usual tendency to expness them with rnuch
present report is based on thein data andgnaphs of the tempenature and dew pointgnadients; they can be seen at the Freibr:rgM{f:s tenbauvere i.n .
4.3. EFTECTS OF REPEATED CONDENSATION - PART 2
In Section 6 of N.L.22 there was adiseussion of the possible effects of nepeatedeondensation, and it was concluded that thisrnight not be as dangerous for the glass ashad been generally supposed. A search hasbeen made of the l-itenatu:re for reports onthis pnobJ-em but no direct experinents havebeen found. However, a comparrison can bemade between two papers, abstracted as No.244in N.L.22 and No.260 in this News Letter.
In Fig.4, on page 43, of No.260 a muchLower attack occurned on the glass when thenewas ilwate:r nrn offt than when ther"e wastrcondensation necyclingrr, and the author"corunents that the reaction products hadprobably been washed away in the second case.However, Fig.3 on page 144 of No.244 seens tosuggest that one monthrs storage in a ltdamp
basementrr foll-owed by 2 monthsr rrcondensationrecyclingtr (curve B) produced 3 to 4 tinres asmuch rrhazert (penrnanent damage to the glasssunface) as did 2 nronthsr condensationrecycling alone (curve A).
If this interpnetation is cornectr then1 rnonth in the damp basenent seems to havepnoduced nore darage (or perhaps initiatedthe possibility of more dannge) than twonpnths of condensation recycli-ng.
Moreoven Fig.1 of No.244 suggests tbat2O9o of haze is pnoduced by 2 rnnths ofcondensation necycling wheneas Fig.4 of No.260suggests that 909o of inaze is pnoduced by 1month of condensation necycling. Tlrus thesituation is rather confused and there iscleanly a need fon one of the well-equippedlaboratories to carny out a proper prognanuneof research i.nto this vexing question.
An important fact br:ought to light inNo.244 is that the higher durability of theoriginal ttfir:e-fi-nishedtr surface skin on newlyunnufactured glass is destroyed by stonage fononly one month in the trdamp basementil.
greater pnecision than they waruant! Butlaynen may weJ-1 assune that the figures areaccurate ones, and I should thenefo::e comnenton the position.
First, thene is the numbe:: of ttsignifi-cantrt figunes which are quoted in the result.Fon examplen two of the laboratonies habit-ually send me figunes fon the sil-ica contentswhich contain four nurnbensn such as 55.69%;there are many examples of these in N.L.2I,pages 10-14! However, the statisticalttconfidence limitsrt fon these silica determin-atjons ane only about 0.3 on the percentage
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3cale (i.e. the limit would be 0.6% of asilica figu:re quoted as 50.009")- Hence founnumbers ane cer:taj-n1y not justifiedn and eventhe third fi.gure would not be conpletelyaccurate because the ttconfidence limitstt a:re
a 0.3 and the conrect figu:re would lie between(say) 49.7eo and 5O.3eo. In Section 5.2 of thisNews Letten I have thenefone nourrded all tbedata to thnee figu::es at the most.
Secondly, some sanples have now beenanalysed mone than once, and tbe nesults arenever exactly the sane! Sornetimes there arequite big diffenences. Examples in N.L.2l-wene analyses 2 and 3; 6 and 7; 13 and I4t99 and 100. But the window glass from DennisHall, in Stounbridge, built in 1770 has nowbeen analysed in four ways. The fi.rst twooccasions wene analyses 99 and l-00 in N.L.21tthe quoted silica contents being 73.3% and68.849o. The glass has been tested again atYonk (analysis No.I49 in this Neva Letten),using ': diffenent colputer Pnogram to pnocessthe results, and the sili.ca content is nowquoted as 67.7%. Howeven, Dr Paul at the-tlniversity of Sheffiel-d has kindly detenmined'the sil-ica content by wet chemistry and heobtained the figr:res: 65.8r 66.01 65.6 and65.7.
It seems clean that the figure of 73.39o(No.99) is wrong but thene is stift a nange of3.4 between the othen results ' which isstati-stically significant. Thus all thefigunes have to be treated with caution,despite the optimistic feeli-ngs of those whosupply them! If any other laboratory wishesto test this sanre sample of 18th centurywindow glass, please l-et ne know, stating thequantity of glass requined.
6 BOOK REVIEW
Most of us who are concerned with theCorpus - as enaftsmen! master-glaziers,scientists, restorens, art-historians, on glasspainters - tend to live in compartments whichare too namow fon our own best wonk. We
genera.l.J-y do not know enough about each otherfsideas, pnoblerns on points of view, and pe::hapstoo often we do not even care about them.This is bad for co-operatj.on and bad for our-selves. We should wel-corne opportunities fonlearning sonething about the other unntsviewpoint.
A book has just been published on I'The
Appneciation of Stained GLasstrby Lawnenee Lee(118 pages, 74 illustrations, including B incolour, ISBN 0-19-2Il-913-3 Oxford UnivensityPness, London, 1977, 95.50) which is venypenetnating, powenfully wr^itten and thought-provoking. It desenves careful reading by allperaons who want to understand the specialpnoblems of stained glass as an art fonm.
Thene are eight mai,n chaptens in thebook: the fnanewonk, the glaziers, thespectaton, scale and stnucture, line, colour -
Thirdly, there are some gaps in the data.Fon example, York Univensity does not have arrstandard glassrr with a known amornt oftitania, and hence thene is always a gap for"T1O2,q
As wiLl be seen from the other analysesin this News Letter and in N.L.21r there isusually sone TiO2 present (sometiures about0.29o but oecasionally as high as 0.49o, as inanalyses Nos. 17 and 2B). The computenptognar in use at Yonk autonatically adjuststhe total to 100.0% and hence the presence oftitania in the glass would rean that a1l- theother results would be given slightly tooIarge a figure.
5.2 CHEMICAL ANALYSES OF MEDTEVAL WINDOW
GLASSES - PART 2
The first 1ist, of 125 analyses ofmedieval glasses, was given in Section 5
(pages 9-14) of N.L.21r and the next 25
analyses are set out in the tab.le here. Thefir:st 17 glasses ane all from CanterburyCathednal, by the kindness of Mr Fneder"ickCole of the Canterbuny G1ass RestorationStudio.
Samples 143-145 supplement the data aboutwindow glasses of different dates, given inTable IV of N.L.l-8. No.145 is the sanre glassas the Buxted Pank sanple in Table IV, but theresults ane slightly diffenent. Samples 146-l4B are fnom St Michaelskirche in Vienna, andcan be companed with Nos.89-91 on p.13 ofN.L.2r_.
Sample No.l-49 is the sanre as thatpreviously analysed (by diffenent l-aboratories)as Nos. 99 and 100.
the rrsoundrr of light, painting and design.Lawrence Lee discusses all of these with greatperce.ptivity and considerabl-e good humoun.(RGN - He does not discuss consenvation but Iknow that he adrnines the oniginal beauty ofan early window and deplores the .l-oss ofcoLoun-balance which can occur with age.)
Stained glass is unique in being theonly ant fonm which depends on the t:ansmissionof scattered light for its portrayal. Forexample, it wouJ.d never be possible to enjoya stained glass window if we were to live onthe moon, which has no atmosphere to scatterthe light from the sun. But there alre alsosevera] othen nespects in which stained glassdiffers nadically fnorn other fonns of ant.It is rrmonumentaltr in size and must conformto the anchiteeture of the building whichhouses it. Ihus no one can have a ftcollectionof windowsrt but only small portions of windows,such as roundels, and this is why display in amuseum is genenally a p6on substitute exceptin rane cases, such as the ttCloistersrr neal:New York, whene the iluseum has been speciallydesigned fon the display of utrole windows.
I2
In addition, a peculiarity of medievalstained glass is that it was virtuallyrestnicted to displaying a single neligion,and even then in a rathen restricted pant ofthe world. It requines a special techniqueof painting, using a loaded bnush used with aconfident single sweep on a slippeny surface Ino touching-up can be done. And an over-riding irnportance was given in redievalwindows to depicting the human appearance andfacial- expression.
Final}yo and what is really obvious, itdepends on the proper use of coLour and of.l-ead lines. These two points are fulJ-y dis-cussed in Chapter:s 1, 6 and 7 wi.th a nicetreatment of trhalationrr and the perception ofa multi-coloured image as a single visualimpact. Al-I of these special chanactenisticshave a beaning on the antistfs approach tomaking a windown and the obsenvers I appnecia-tions of it but, in addition, the genenalobserver encounters some extra difficulties,
7 ABSTRACTS
25L, BECKSMAN\ 3. (1976) ilPr"obleme de::Re stafrier:u@i s che:: Glasmal-ereien I'
(Problems in the r€storation of -l-ate-Romanesquestained glassil A neport on the Colloquiumheld in Linnich on 29th and 30th Manch, 1976,published in Kunstchnonik 1976 29 330-337.
At the time of the Colloquium the chancelglass from BUcken on the Weser, the Jessewindow fnom St Patrokii in Soest and parts ofthe Jesse window fr:om Fr:eibung Milnster weneat the Oi-dtmann wonkshops in Linnich, andFneidrich and Ludovicus Oidtmann were preparedto give a demonstration lecture.
After a genenal tour of the workshops,the 30 participants discussed the work inhand, whi.ch pnovi-ded an excel-lent basis forthe discussions of the second day. U.-D. Kornreported on the critical condition of theBiicken glass and the difficulties of restor-ation. G. Frenzel had circulated a report onthe state of the Regensburg Jesse window, withcriticism of the wonk being carried out atRegensbung, and U.*D. Korn gave an introductionto the problems of nestor:ing the sevenelycomoded Jesse wjndow from Soest. A concludingpaper was given on the condition of theFneiburg Jesse window and the plans fonrrde-r:estonationri.
G. FrenzeL used photographs to demonstratethe deter:ionation which had occurred in theRegensburg Jesse window since the thirties.Since 1940 almost all the half-tone work hasdisappeared, and the linework is now in greatdangen. This applies both to the black enameland to the flashed r:ed in the borders - aremarkable technical- feature. This wasobviously due to the extr:enefy high level ofmoisture in the atmosphere of Regensburgcathedral. He had submitted pnoposals forrestoration in 1974 which wer:e discussed by an
such as the afmost complete anonymity of theartist, a genenal l-ack of information aboutthe craft of using col-oured glass, and an evenmore nemote rinderstanding of the making ofcoloured glass (pot metal, not rrstainedrr
'g1ass') .
The interesting ilJ.ustnations have beencarefuJ.ly chosen to illustr:ate alJ- thesevarious poj-nts (although, in my view, page 27does not do justice to the Great East Windowat York Minster', and I should have tikedtheChagall window at Tudeley to have been incofour because the interplay between bl-ue andbl-ack in the original is completely lost onpage 42). Moreover:, in Chapter" t he does nothesitate to ill-ustrate and discuss some badexamples of l-9th and 2Oth century glass.
There are a few minor errons in the bookbut the only one I will- remark on hene isthat the Five Sister"s window at Yonk is datedto about l-260 and not I40O as stated on page23.
international- committee of expents which::ecommended isotherrnal extennal protectivegJ-azing, which would provide pnotection with-out interfering with the glass. An aestheticcompnomise had been attempted in that theexter-na.l- glazing had a simulated leaded appear-ance but this was not reafl-y successful-. Ifthe relative atmosphenic hunidity in thecathedral wene more than 609o0 then the heatingof the window zone, suggested by Frenzel, wouldbe absol-utely vital to suppress furtherweathening. The specific interior cJ-imatewould have to be measured over an extendedperiodo as at Freiburg M{lnster (see Section4.2 above). The committee al-so necommendedItemer:gency fixing of the bl-ack enamelrr, butthis is a term l-iabl-e to misinterpretation.According to E. Frodl-Kr"aft it is not a stop-gap nor an econoTrs/ measune but irnrnediateactj-on to fix loose black enarel- by saturationwith a sJrnthetic resin solution after cleani-ngof the glass surface. A description of thewonk invol-ved makes it obvious that the ideaof treating l-OO m2 in thnee weeks is quiteabsurd. The revensibility usually demandedin such cases was cl-aimed to be quite sense-less in this particul-ar case, arrd on thispoint there was complete agreernent. Fixingbl-ack enanel with slmthetic resin is a processwhich, regandless of whethen a single or two-component adhesive is used, cannot be reversedwithout rnaterial 1oss, and this places thehighest demands on the skil1 of the nestonen.For this reason, it is not possible to canryout provisional fixing of l-oose black enamelbefore the glass is dismantled or transportedto the workshop.
In the light of the Biicken example itwas obvious to a.ll that the cleaning of theglass surface before the fixing of the bLackenamef is fraugirt with gr"eat pr:oblerns. Thethnee chancel windows of about l-250 f::om
fo
St Materniani (see Hans Wentzel-:Niedendeutsche Eeitrdge zun Kunstgesctrichte )wene in quite good condition when stonedduring the second wonld war. In late 1975 theywene again dismantled fon the installation ofpnotective glazing. Fon neasons of cost itHas not at finst planned to canny out reator-ation wonk. However, caneful examination atthe Linnich r,o::kshop showed that they couldnot be re-installed in thein existing con-dition. This can be seen fnom comParison ofphotographs of the conditi-on at the pnesenttime (Fig.Sa) ed that in 19a3 (Fig.Sb).
A pa:rtieularly wonrying featune is thecondition of the inside, possibly caused byincon:rect storage during the war and by con-densation, which has alneady led to signifi-cant loss of black enanel and has evenextended to unpainted areas. There a::e alsomould-1ike depositsn Particularly on the heads,with the affected aneas sonetirres completelyopaque (Figs. 5a and 6a). Such pbenonrenahave not been encountered anywhere else andthey nequine caneful chemical investigationbefor.e treatment, on which a start has been
': made. There is a1so, especially on the flesb-colouned areas, sorne bnolning of the inside -this is fneguently encountered on nredievalglass but it has not yet been satisfactorilyexplained. In addition, the luminosity andtranspar€ncy of the originat pants have beenimpained by sever:e cornosion of the outside,in the form of scaling of the glass. Thisproeess has already penceptibly r:educed thethickness of the glass and almost completelydestnoyed the external paintwonk. The severity
.of the comosion is surpnising because BUckenis not an industrial anea and it was suggestedthat unden ce::tain weathen conditions theatmospherj.e pollution of the BieJ-efeJ-d andOsnabn0ck anea 100 km away might drift thatfan.
Difficulties al-so arise from the partsrestor€d in 1866, whieh rnake up none than haJ-fthe totaL. . The techniques available at thattime r.,ere not good enough to give adequatefusion of paint and glass so that a majon parthas come auay because of condensation. Thisspoils the ovenall picture and also neans theloss of a histo::ic document, because thereatorer had copied fragments of the originalwlrich are now l-ost. It was thenefone decidednot to neplace Welte::rs technieally inadequatenestonation wonk but to find ways of preserv-ing it, pnobably by putting the lost paintwonkon to cover glasses in the case of the figuresand by cold painting on deconative Pants.
Al-l- the participants agreed that caneful-consenvation and restoration of both theor.iginal and the 19th century work was amatter of ungeney. The following steps shouldbe taken straight away: nemoval- of looseweathering crusts on the outside whereventhese severely impain the transparency andnepnesent a source of furthen destnuction;caneful cleaning of the inside and fixing ofthe loose btack enarnel; neplaeing lost paint-wor:k on the t9th century glass. The partici-pants lient so fan as to addness to the r€levantauthonities a nesolution wi.th detailed
suggestions, which as fa:: as the original glassis concerned have alrready been met. Anylasting success at B{lckenr howeven, will onlybe achieved if the re.l-ative atmosphenichUuri.dity in the vicinity of the painted glass
" can be kept below 60%. In this connection,the question of supervision was also naised.
Quite diffenent problems were :raised byKornls report on current restorati-on wonk onthe Soest Jesse window; probably dating frombefone 1166r it was reinterpneted as a Marywindow by additions by Baudni in l-863/64.Uniform weathering cr.ust had formed as ea::}yas 1897, possibly because Baudri had etchedthe old glass on the outside. Photos in J-943showed va:rying degrees of opacity, a negativeeffeet being pnoduced with most of the l-ine-work resisting attack. Dantage had been causedby storage in a ce.Ll-ar which was sonetirnesunden water. Amateunish restoration workafter the war had sacrificed almost all themedieval l-eadwonk. Since then efflorescenceand soot deposits have r:educed the window toa nuin. It has therefore been decided tonemove the ,Jesse window, afte:r careful-cleaning and fixing, to the Rfstkamner Museum,which alr'eady holds the remaining parts of themain chancel windows, and to replace it by acoPy.
In Fneibung, on the other hand, thene isreJ-uctance to nemove the pre-1218 Jesse windowto a museum, even though the nine nedalfionshad a}:eady been noved from their oniginalposition and had thus l-ost mueh of thei::impact. Basieally, the opi-nion was that allglass should be left in its original pLace,insofan as this is possible from the point ofview of conservation. To this end, since 1971there has been a Planned instaLlation ofexternal protective glazing, which is furtherassisted by a nelatively dry, balanced indoorclimate. Itre windows of the south transeptuere given pnotective glazing right at thebeginning of the ten-yean pnogramme, butrestonation of the Jesse window was postponeduntil expenience had been gained from work onthe mone necent glass. Fortunately the delayhad.no adverse effects. Since 1967 the stateof r.ieathe::ing both inside and out has remainedalmost constant, thanks to the externaLpnotectJ.ve glazing and the dry indoor climate.The Freiburg Jesse window does not theneforepose any special problems.
Although the so-cal-led weathening crusthene had, lJ-ke Soest, been removed mechanic-aIly in the 1870s and the hal-f-tone workpossibly existing at that time had beendestnoyed, .the relatively slow funthendeterioration of the outside is probably dueto the above-avenage quality of the colouredglasses. Inforrnation about weathening in thepast 60 yeans is provided by excellent photosbefone and aften Geigesr nestonation of 19J-1,showing that Geiges only removed weatheringcnust from the outside when it had begun topeel away by itself. In addition, Geigesused cold paint over: any pieces which seemedtoo bright, such as the unweathered greenglasses. He justified this by neferences toVi.ollet-1e-Duc, who claimed that the J-3th
14
century glass painters applied a cold gfazeto certain glasses in orden to balance outemors in eolouring. However, since thiswork significantly altered the artisticimpression, this is a case where ttde-nestol-
ationtt should be welconred. In view of therelatively good state of the black enamel,rernoval oi th. cold over-painting with an
acetone solution would not seem to be toodifficult.
It was agreed that the basic rule shouldbe - Maximum preservation by means of minirnum
intenference r.dth the original rnaterial' As
fan as Regensburgr Soesto Btlcken and Freiburgare concerned, the followlng technicalproblems dominated the discussion: thepnoblem of fixing the bl-ack enamel, stillunsolved after fifteen yealrs. However un-satisfaetory the fixing of loose bJ-aclc enameL
by saturation with an Analdite o:: Viacrylsl.l-ution nr-ight be, no better method is avail-abl-e at present. Just as iinpo::tant are thesense of responsibility, manuaf skill and
a::tlstic "uttiiti,tity of the restor€r' Special
':problens are raised by the extensive.restor-ation which has gone on since the middl-e ofthe last century - some want to remove al-L
such insentions while there is a slow npvement
toward.s respect for such viorkr aimed atintegrating it into the whol-e' Discussion ofthesJ problems pr:oved very fnuitful and there
""" u
-g"rr""al- desire for" more meetings ofthis kind.
252. BRILL. R.H. (J-963) "Analyses of
"or. ffid" F6il-tfieT"a1i6 wneckrr J. Gl-ass
Studies i-963 15 93-97.
This is the third paper of a series on aconnected theme which are printed together'The first Paperr by Astone Gasparetto (pp 79-
84) describes the discoveny of a l-6th centur5/wneck off the (Jugoslavian) island of Gnal-i6
and suggests that it was the ship 9agignawnicfr had left Venice for Constantinople on
lO November l-583 vtith a heavy mixed cango
which inctuded much gfass. Some of the glasswas fl-at (eg window) glass vtrich ttstifl had
straw between the layerstt after sone 380
The second Papenr by Sof.ija.Petriciol-i(pp 85-92) describls the 86 types of glass."ti.t"" n with of course special attentron tothe decorated domes'tic glassware' 0f theflat g1ass, there were 648 nound window panest
iog.t["" with'ttinnumenable fragmentsrr' of the
tol, zOS are intact and the nest are slightlydamiged. They are of 3 sizesr 2O5t l-85 and
170 mn in d.ialneterl several are il-1ust::ated'The sinplest pieces of fl-at gJ-ass were
iruegular: in shape, of vanying sizes nangingfrom 22O x lBO mn to 300 x 380 mm' None ofthe fl-at glass was colouredn although sone
of the vegsel glass was blue and the glassbeads vere wtrite, blue or broun'
Six of the pieees of glass were analysedby the Cor:ning Museum of G1ass and theairalyses forn the subjpct of the third paper'one tf the six samPles was from one of the156 mm pieces of circulan glass but al-l sixanalyse! rrane generally quite uniform in
composition, so much so in fact, that allapp"ao to have been made in the sane factorytthl window glass being made in the S349-$ggYas the vessel glassrr. The analysis of the
.r^Jihdo:!,r glass is:- SiO2r TI%; Na2O, 13'7%;
CaO, 6 .53%; K2O , 2.BBeo; MgO, 1' 76eo;
AIe6q, 1. 60o,; -Fe2O3r
O .75eoi IhO, O .94eoi
Pb6r"O.28eo; SrO, 0.l8eo; the next largest.ott"titn".ri was Sn02, O.OB9o1 arrd seven othersare quoted. This composition isr-however,not Jt all l-ike any of, the compositions ofmedieval window glasses, quoted on pages 10
to 14 of N.L.21.
253. DMCHENBERG. E. (1975) rrPnobLeme
de n E;fr-al-tun g rm d Res t aurie n:n g mi tte f al-te r-Iicher Gl-asmalereir? (Problerns of preservingand restoring medieval stained glass)'Denkmalpflege in der DDR 1975 Z 38'47 '
The author explains the weathering ofmedieval- glasses (especially 14th centuny) interms of ift.io ftigtt alkafi contents, and thendiscusses in general terrns the cleaning,fixing of loose paint, and conser:vation ofthe cl-eaned glass. He advises that commercial-products r with wel-l--defined compositions o
shoufa be used, and that all methods shouldbe reversible.
He considers that by far the most diffi-eult job is fixing the paint; it eannot be
refired but coatings in the manner of fustreg3.azes are considered to be suitable ' A
iolution of metal soap in oi-L and turpentineis applied with a paint-bnush. After dryingtrrburning inrr is carried out at temperatur€sof up to 25OoC in an oxidising atmosphere'The Lrganic comPonents volatilise, leavingthe oxide cor:responding to the netals used -al-umina and bismuth oxide have proved suitabl-e'Both are colounless, have no optical effectsand form a lasting protective layer' Tbisd,oes not solve the problem of the half-tones,vihrich get darker and spoil the effect' Alsotheating to onl-y 25OoC represents a threat tothe glasses. Reseanch has not yet yielded a
definitive nrethod of fixing black enamel'
Cleaning of the glass and fixing of thepaint must always precede any process of-preservi-ng the glass itsel-f if thene is to be
any hope of success. Various methods have
leln triea, such as plating llith or withoutan interJ-ayen of synthetic resin, but itproved to be r:nsuitable in the long term -Litn." because it discoloured or the paintworkwas damaged when removal becarne necessarli 'Or renewed penetnation of moisture took Place'
He also discusses the use of protectivenesins (which cannot always be removed withoutdamaging the glassn as has been claimed);tttu ipnitu"ing of inorganic glasses (the glasswas n;t coated uniformty and it is sti1lsusceptible to corrosiono but the coating isinvisibl-e and the rnethod rfght be improved);isothennnl gLazing is therefor:e recomrnended
as the best available prophylactic pnocedure'He discusses isothermal glazing in some
detail (pp +r+ end 45) and refens to thewindows .l li.,a.tt. (see page B of N.L.7),
pointing out that they were not ventilatedalthough the resul-ts (after Eld-years) anefavourabLe. He considers that the extennalwindow shoul-d be made of laminated safetyg1ass, as a protection agai.nst stone throwing,and that it should be cut up and leaded 'rinthe form of rectangles or diamonds whichshould not be too smallrr. The medieval glassshoul-d be hung 50 to 100 mrn in fi:ont of theexternal window and be stabilised by means ofcopper frarnes; he discusses three differentways of doing this. The necommended widthsof the ventilation slots are 10-20 mm at thebottom and 20-30 mm at the top.
0n page 46 he lists 21churches and cathednal-s inisothermal glazing has been
254. ERNSBERGER, F.M.glass by chelating agentsrrSoc., 1959 42 373-375.
churches, villagethe DDR in whichins tal-led.
(1959) rrAttack ofJ. Ame::. Cenam.
The author studied the attack on gl-assof chelating agents (agents such as EDTA whichcombine with insoluble materials, such ascalcium compounds , to form so.l-uble compounds ) ,appanently with the intention of testing thematenials for. etching glass instead of r:singthe rather aggressive ammonium bifl-uoride onhydrofluor"ic aeid. These agents will alsoattack the weathering crust on glass andhence the paper is of interest in connectionwith the cleaning of weathe::ed gIass, but itwas found that EDTA will attack the glass topnoduce a surface rrindistinguishable fr:omsurfaces etched to a compar:able depth byhydrofluoric acidrr. Hence the surface wouldseem likely to be liabl-e to furthen attack bythe atmosphere.
The authon studied two chel-ating agents(sequesteri,ng agents) wittr quite differentfr:nctions, EDTA and Catechol. EDTA willconrbine with calcium, magnesium and aluminiumto form sol'dble eompor:rrds, and it forms thebasis of J.M. Bettembou:grs rrSol-ution Btl(see abstract l-81 in N.L.13 and Section 2.4of N.L.7). Catechol- will forrn a sol-ublecompound with silicon ions, and hence itmight prove to be a useful additional neagentfor removing those weathering cnusts whichcontain silica. It does, however, have thedisadvantage that it combines with oxygen andhence any cleaning of glass with it wouldhave to be done in an inent atmosphene.
The papen sbows tLrat the cheJ-ating prop-e::ties of EDTA and catechol were additive.The optimum concentrations were 0.2% EDTA and0.4% catechol, and thein effect was greatestin an alkal-ine solution. The maximum nate ofattack was 12 fi:inges per houn at 100oC (-say2mm per: month), on about 1/300th of the nateof attack of l-mol-an ammonium bi-fluonide.
255. FERMZZI.NI., J.C. (1976) 'rVorteileuna rffinikEffiff1on Cyanoac::y1at-Monomer.-Kl-ebstoffen zur: Sichenr:rrg der Maler:eivon Glasgemdldentr (Advantages and techniquesof applying cyano-acryfate monomer adhesivesto protect the paint on stained glass ) .Glastech.Be::., 1976 49 264-268.
The authon points out that in sorne casesthe base glass has weathered whrile the paintedlines a:re dunable, but in other cases (all-ill3str:ated) the paint has weathered and iskept in place only by static charges and itcan easily fal1 off. The Epoxy resins so farused have two disadvantages, they are notreadily r:eversible and aire visible to thenaked eye;
Any fixing matenial fo: use wh"en removingwindole fon transport must be napid in itseffect, safe, easily neversible, invisibl-e indaylighg have good corr:osion resistance, beeasil-y diluted with safe solvents and beeasily applied with a very fine paintbrush.About 20 natenial-s were tested, and cyano-acrylate monomen adhesi-ves wene found to bethe best. Four: diffenent comnercial brandswer:e used and the best was found to beCyanolite 201- (from Minnesota Mining Products).
Full details ane given for applying thediluted adhesive, depending on r.'rhethe:: athick weathei:ing cnust is also pnesent inwhich case the crust should fi::st be saturatedwith a toluene-acetone solution. Othenfeatunes to be considered are whether corrosionhas attacked othenwise sound paintwonk fnomthe side, etc. The paper (and/or No.256)shoul-d be consulted by anyone proposing to usethe technique, because the Table on p.267discusses the sour"ces of enror and methods ofovercoming them. The hardened adhesive isremoved with dimethyl formamide.
256. FERMZZI"NIT J.C. (f976) "DieAnwen dun g von Cyanoa crylat-MononrenrKl-eb s tof fenin der Gl-asrestaunierungrr (The use of cyano-acryl-ate monorrer adhesives in nestoration ofglass). The info:rmation Bull-etin of theassociation of museums in Switzer.l-and 1976r_6 s -10.
This anticle is simil-ar in content toNo.255 but it also deals with mending cracksi-n glass. Cyano-acnylate monomer? adhesivesare colourless, rapid setting, invisible onthe glass (refractive index = 1.451-7), easilyr:eversible using N.N. dimethyl fonmamide,and no surface preparation is needed. As faras cracks in the g.lass are concenned,Cyanolite 2Ol- is recornrnended fon cleanfr:actures only; old fr.actures with cnumblingedges should still be mended with epoxy resinadhesives because cyano-acrylates shoul-d beused in thin J-ayens only (they form a su::facelayer which prevents the material underneathfnom dnying out).
Details are given of strengths of bondsand the::e a::e six illustrations showing how touse the matenial. When fixing loose paintthe adhesive sets in a few seconds. If alonger setting time is r:equir:ed, Cyanolite2O2 is better:. The shelf-life is l-imited toone year at most when kept at 5oC; at -20oCit is unlimited.
0n pages 7-B ther:e is a discussion ofreversibil-ity because E. Bacher has expnessedhis doubts about nevensible pnocesses duningwhich a paint fnagment may become l-ost alto-gethen (Linnich Symposium 29,/30 l'Ianch 1976,see 251 above) . Fercazzinirs solution to this
16
Cyanolite so that there is a cont::oflablereversibility; he for:nd that 5-1Oeo of
blem is to add a little Araldite to the after which the rrreather:ing cnust is nenrovedby mechanical means. Details ar:e then givenof the various changes made in the window andrestoration pnocedunes such as fir:ing-in
"r#S"Ti'tg paintlrork. Mending leads ar:e nemovedwhere possible and the cracks nepaired with a
special adhesivel these cracks are stabitisedwith lead pl-ugs at intersections. (Lediglichan Kr"euzungspunkten wenden die geklebten Rissedurch Bleiplomben stabilisiert.) The outenedges of panels are str€ngthened by tinsolde::ing and inserted in a copPer frame, butthis is complicated by the fact that everypanel is of a different size so that a specialfname has to be made for each of the l-50paneJ-s.
260. SIMPSON. H.E. 'rMeasuning surfacedurabTTty ;f-ej-faffBill . Amer. ceram. soc.19s1 30 (2) 41-s.
Abstract No.244 in N.L.22 desc::ibes theresults of many experiments on the tneatmentof glass sunfaces, using a technique formeasuring the |thazetr (permanent surfacedamage) produced on the gfass. This papergives fuIl detaits of the apparatus forcarrying out these tests.
Some test results are al-so included;Fig.3 shows that more damage is caused toglass by repeated condensation and d::yingthan by continuous condensationr apparentlybecause the .l-atter situation pr:oduced largeamounts of r,rater: which ran off the su::faceca::::ying the reaction Products with it.Fig.8 shows how two modenn window glassesgave quite different results because they haddifferent compositions. One was more thantwice as durable than the othero and itcontained 12.I% of soda whereas the lessdurable one had I5.29o of soda (the fullcompositions alre given on p.45). Four opticalglasses (compositions also given) behaved inquite different ways when subjected to thetest.
26L ru4!t4!{r_Rao (1976) rtStness-..-.-corrosron resrsrance compared with thermalexpansion and chemicaL durability of glassesrlGlass Tech. 1976 lJ 145-6.
The author is primarily concerned withpredicting the stress-corrosion-resistance ofgl-."="" (which is tedious to neasure) and he
points out that it is correlated with thecoefficient of thermal expansion of the glassand with the chemical durabi-Lity (towardswater) o which are conr:elated with each other'RGN - the paper is of interest fon the theoryof conservationn because it would indicatethat the poorly durable medieval- glassesautomatical-ly have a relatively lange thermafexpansiono which might make it difficult touse Acloquers pnoposed conservation process( see Section 2 . 3 .5 of N. L.2 3 ) . Medieval-glasses are bel-ieved to have a coefficient oflxpansion of twice that of modern glass (see
Section 3.6 of N.L.1B). Acco::ding to thispapelr, the medieval glass wouJ-d have one
tenth of the dunabil-ity of the modern glass'Here again, the glasses in most ungent needof protection wilf be the most difficult ones
to protect.
Ar:aldite (Aw106/HVg53U) in cyanolite 202ficient.
257. NEWTON. R.G. (1976) I'First neportto ttr--noyaTEffi6n cleaning staineoglass with the airbnasive equipnentrr 26 pagesof typesc::ipt and 60 colour photographs.
This Seport studied the use of varioustypes of particle in the airbnasive apparatusut a it was found that the harde:: types of gritsuch as Nos 7 arid B (silicon carbide) couldeasily <1an'nge modern glass, producing a cavityO.Smm deep in one second. The afurninapowders (Nos 3 and 5) produced less damage and'the hole made by No.5 powder was only 0.12mm
deep after 4 seconds, pl-enty of time fo:: thecraftsman to stop the jet after the cnust hadbeen removed. Powder" No.9 (glass beads) didnot produee neasunable damage on glass evenafter 8 seconds. The hardest weatheringcrusts carr easily be :removed using alumina
..powders, and even with glass beads if the'crusted glass has first been kept in a drystore for sore weeks before cleaning starts.
Some accelerated weathering tests werecarried out on the ai::br"aded surfaces, and
they failed to confirm that the cleanedsurfaces are more susceptible to weatheringthan were the originalo uncleaned su::faces.
258, POLAK, Ada (l-975) n'Glasso itsmakens and its pu-nficrr Weidenfeld & Nichol-son.London. 97.50.
This wel-l--documented book desc::ibes glassand glassmaking, fnom medieval- times to 1B7Ot
with discussions of all its interactions as
determined by geography, economy and fashion.Its scope is best described by listing thefirst seven chapter headi-ngsl- J-. Glass andglassmakers. 2. Glass and the Church.3. Glassmaking in forests and tovms. 4. For"estglassmaking in the north. 5. Medite::raneangJ-assmaking in fo:rests and towns. 6. Venice,the greatest of ur:ban glassmaking centres.7. The italians introduce glassmaking in thetowns of the nonth. Chapters B-22 deal- withdecor:ated gl.ass or with post-medievaldevelopments.
In the USA the sane book has a differenttitle:- ilGl-ass - Its tradition and its makersrr,published by G.P. Putnamrs Sons, New York,$ls .95 .
259. SCHMID-I-L Chrigta (1976) "zunRestaiETerlrfr -ffi-aletreieninMiihLhausen'l(The restor:ation of stained glass at MUhlhausen)Denkmalpflege in der DDR' 1975 2 42-5L.
Descniptions (and iLlustrations) ane givenof four panels from the Marienkinche. Tlrese
had been Left in place during the war andsurvived without mueh damage occurring. Butthe medieval- glass fnom Blasius chu::ch was
r:emoved du::ing the wa:: and stored in a cellartand it is now completely destnoyed.
In nestor:ing the Marienkirche windoustany l-oose paint is fixed with Piaftex solution
LIReproduced by the University of York Printing Unit
