7thINTERNATIONAL CONGRESS

ON DETERIORATIONAND CONSERVATION OF STONE

PROCEEDINGSVOLUME 3

--,

Lisbon · Portugal, 15- 18 June 1992

Edited by

J. Oelgado Rodrigues • fernando Henriques • F. Telmo Jeremias

_ boratorio Nacional de Engenharia· Civil~ _~='== ~-::::. . ~_ J

Organized by

CONSERVAMON O FERRUGIN SANDSTMa WED H NORM-WIM DIM"

©amengat n d .2efrughneux rIljaá flral e ta 1grquis

EDDY DE wi-7EHead «f Department, Raayal institute for Cultural Heritage, Jubelpark 1, E-1040 Brussels,Beigi um

KAREL BOSArchitect, Royal lnstitute for Culture! Heritage, Jubelpark 1, -1040 russe ➢s, Belgium

SUWMARYThe effectiveness of freatments of ferruginous sandstone with c nsolidants based «n ethylsilicate and oligomeric methyl siloxanes water repelients w s investigated. Consoiidation ispossible by a two or three foid application. Abrasion resistance increases in en import nt waywithout any negetive influence on the drying r te. Hydraaphobizatien protects ffectivelyagainst acid rein and 1ichen growth, but in a minor way against acid atrnosphere. Artificialageing prove that the treatrnents wilt resist for at least one or twaa decades, but that careshould be taken not to apply the hydrophobic agents on st«ne contai ing saaluble salts.

RÉSUMÉOn a recherché l'efficacité des traitements du gres ferrugineux par consolidatie n vec desproduits á base de silicate d'éthyle et d'hydrofugation avec des oilgomeres de siloxane deméthyle. La consolidation en deux aau trois traiterrerits successifs donne de bons résultats.La résistance á l'abrasion augmente d'une maniere importante sans effet négatif sur !auitesse de séchage, Les hydraafuges protègent efficacement contra pluies acides et leslichens, mais la protection contra ure atm.sphère acide est faible. Des vieillissements artifi-ciels prouvent que les traitements résisteront au mois me au deux décennies, mais qu'il nefaudra jamais appliquer des hydrofuges sur d s pi rres qui contiennent des sets solubles.

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1. INTRODUCTION

In a previous paper [1] the quarrying, use and degradation offerruginous sandstone as historical building material incentral and north-east Belgium was discussed. Fresh and agedsamples of two formations (Brussels and Diest) have beenanalyzed and the physical and hygric proporties measured. Fromthe obtained data conclusions could be drawn concerning theresistance to air pollution and frost resistance. In this partthe effectiveness of consolidation and hydrophobization ofnaturally weathered samples is investigated. For theidentification of the samples investigated in this work, thesame code as published in [1] was used :B : Fresh ferruginous sandstone from the Brussels Formation,

originating from the sandpits of Chaumont-GistouxDF: Fresh ferruginous sandstone from the Diest Formation,

originating from the Area of AverbodeDO : Recuperated ferruginous sandstone from the Diest

Formation.If DF or DO is followed by an index, this refers to a quarryor particular monuments the samples were taken from.

2.CONSERVATION

2.1. Consolidation

The consolidation of the decayed surface of building materialswith products based on ethyl silicate is common practicenowadays. It is, however, also known that the chemicalcomposition of the consolidant, the nature of the buildingmaterial and the application technique play an important rolein the final effectiveness. The evaluation in situ of theeffectiveness is stilt problematic as most techniques aredestructive or difficult to achieve after the restoration ofthe monument. Therefore, preliminary laboratory investigationscan give useful information.

2.1.1. Consolidation of powders.

The ultimate state of degradation of a building material isthe decomposition into a powder. So whenever the effectivenessof a consolidant or the feasibility of a consolidation has tobe investigated, it is common practice in our laboratory toexecute tests on powders with a granulometry which is similarto the one obtained by natural ageing. Therefore the granulo-metric curve of a series of powders, collected on surfaces ofmonuments, was determined. As can be seen in figure I thedegradation material from Brusseliaan and Diestiaan between 53and 1180 p.m is very similar, except for the region between 125and 500 p.m, where the fraction for Brusseliaan is considerablyhigher. For the consolidation tests, naturally aged stone fromboth formations was crushed and powders with an identicalgranulometric curve were composed. Plastic molds of 2.5 cmdiameter and 5 cm high were then filled with the powdermixture and the bottom was brought in contact with theconsolidants, which can then be absorbed by capillary action.The time needed by the consolidant to reach the surface ofthe powder was measured. After impregnation, all samples werestored in a controlled environment (25 ° C, 50 % R.H.}. After 241114

h 50 % of samples were treated a second time. This wasrepeated twice. In this way a series of samples was obtainedwith 2, 3 and 4 consecutive consolidation treatments. For eachapplication the time necessary to obtain a completeimpregnation was noted. The test were executed with twocommercial products : STONE CONSOLIDANT OH from Wacker (SOH)and TEGOVAKON V (TEGV) from Goldschmidt. Both are 75 %solutions of a prepolymerized ethylsilicate in eithermethylethylketon or white spirit and both are catalyzed by anorganotin compound. The impregnation time for SOH was 4minutes for the first impregnation and 15 minutes for eachfollowing impregnation. For TEGV the first impregnation took 9minutes, the following ones 15 minutes. After 4 weeks ofhardening the samples were removed from the moulds and if aconsolidated block was obtained, the weight increase wasmeasured. As can be seen from table I, successive treatmentsresulted in weight gain. During the lst treatment, this was ofthe order of 25 % . A second and a third impregnation resultedeach time in an increase of about 10 %, a 4th treatment seemedto have been less effective as only 2 - 4 % material wasabsorbed. All samples were well consolidated and could beremoved as solid blocks. In this way it is proved thatsolutions of ethylsilicate are potential ferruginous sandstoneconsolidators.

2.1.2. Consolidation of unaged blocks.

The treatment with ethylsilicate changes the capillarystructure of a building material as a result of theprecipitation of siliciumdioxide gel. The purpose of thetreatment is of course to decrease the capillarity of thealtered zone in order to obtain a structure which resembles asclosely as possible the original material while at the sametime leaving the sound stone untouched. In practice this isnot always possible and moreover the underlying, sound layercould absorb ethylsilicate. This can result in a harder layerand if this layer slows down the evaporation rate of absorbedwater, frost damage could occur. It is difficult to evaluatethe influence of such a phenomenon as the determination ofphysical and/or hygric properties is carried out oninhomogeneous samples. Indeed, in a treated sample one canhave following zones : aged and treated; non-aged and treated;non-aged and non-treated. In order to evaluate the influenceof a treatment on an unaged sample, some sound samples werecompletely impregnated with both consolidants and afterhardening some physical properties have been measured andcompared with blanc samples. As can be seen in table II theporosity accessible to water and the saturation coefficient ofall samples decreased after treatment, but the drying rateincreased. This enables us to conclude that a treatment withethylsilicate does increase the resistance of the materialagainst frost damage and that deep impregnations do not have aharmful effect.

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2.1.3. Consolidation of naturally aged samples

Samples of naturally aged ferruginous sandstone were collectedfrom different monuments and treated with SOH and TEGV. Bothconsolidants were applied by spraying the naturally agedsurface for 30 seconds. By weighing the samples before andafter treatment the consumption was calculated (table III).Depending on the original porosity and the state ofdegradation the average absorption of ethylsilicate variesbetween 1100 and 1800 g/m 2 . Extreme values of 650 and 2700g/m 2 however were measured according to the porosity of thematerial. When consecutive impregnations were executed,roughly the same amount of product was absorbed each time, buteach impregnation took considerably more time than the former.it is also notewortyh that in general the consumption of TEGVis lower than this of SOH. The difference between bothproducts is however less important than the differencesnoticed between different samples. After 4 weeks of hardeningthe abrasion resistance was measured and compared with theresults before treatment. The same drilt technique asdescribed earlier was used [2]. As can be seen in figure II, adistinct improvement in abrasion resistance was obtained. Inuntreated samples, the drilt penetrated up to 2 — 3 cm in thesample after 2000 revolutions. After treatment the penetrationwas reduced to 2 — 7 mm. The increase from 2.5 to 7.5 mmbetween 600 and 700 revolutions for sample DO 4 is due to anextremely soft sandy part at the location where the test wasexecuted. It was established that this method only works wellon samples which were not too inhomogeneous to start with. Ifthe stone contains thin layers of limonite alternated withsofter sandy parts, it is very difficult to evaluate theeffectiveness of a treatment. Each drilltest has to be checkedby a visual inspection of the sample. In this case evaluationsin situ will be extremely difficult.

2.2. Hydrophobization

The treatment of a building material with a water repellentprovides a protection against biological and chemicalaggression for many years. Two commercial water repellentswere applied : Wacker 090 and Tegosivin HL100 (Goldschmidt).Both are methylsiloxanes and have been used in a 7 %concentration in white spirit. The products were applied byspray.By using longer or shorter application times, the impregnationdepth was monitored between 2 and 10 mm. The consumptionmeasured during the treatments is listed in table IV. Here aswell the consumption of TEGV is slightly lower than that ofSOH. One week after the application of the water repellents,the water absorption coefficient (WAC) and drying rate (DR)were measured. The same techniques as described earlier wereused. None of the samples absorbed any water during the 15minutes of measurement. The DR however dramatically decreasedin all cases, as shown in table V.

3. ARTIFICIAL AGEING

In order to evaluate the long term effectiveness andeventually negative effects of the treatments, a series of

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artificial ageing tests was carried out.

3.1. Lichen

It is known that lichen growth can damage building materials.As, however, lichen grow very slowly and only in well definedclimatic conditions, specific for each species, artificialageing testing becomes extremely difficult. Therefore stonesamples were treated with a number of products secreted bylichen and commercially available.A small amount (0.03-0.09 g) of following products wereapplied on non-treated and on hydrophobized samples of 50 x 50x 6 mm : B-sitosterol, adonitol, vulpinic acid, D-manitol,usnic acid, lichenan and oxalic acid. After application of theproducts, they were covered with a few drops of distilledwater and then the samples were placed in closed containers inwhich the R.H. was kept constant at 95 % . After 60 daysfollowing effects were registered :- B-sisterol and adonitol did not cause any visible damage.The untreated samples, however, were completely covered by awhite fungus, the treated ones were intact. This phenomenonwas exclusively linked to these products.- Lichenan caused minute brown chips to detach from both thetreated and untreated samples.- D-mannitol recrystallized into very large crystals on thesurface of both samples without causing any damage. As thisproduct is secreted in nature underneath the surface of thesubstratum, the formation of large crystals could cause damageto the rock.- Usnic acid and vulpinic acid did not show any visibledamage. An SEM investigation, however, showed the etching ofthe grains by usnic acid and the formation of small crystalsin the pores of the samples treated with usnic acid.- Oxalic acid caused a dramatic deterioration of the samples(fig. III). Especially the treated Brusseliaan samples showeda distinct discoloration, which also occurred in the othersamples, although less marked.

3.2. Acid rain

Hydrophobized and untreated samples with an average surfacearea of 0.00663 m 2 were soaked in a sulfuric acid solution ofpH 2. After 7 hours the samples were rinsed with distilledwater, dried for 17 hours at 60 ° C and weighed. The cycle wasrepeated 28 times. During the first hours of the test allsamples show a remarkable weight loss, which decreasesdrastically after 7 hours. From the graph weight loss versustime the protection of water repellents against liquid acidscan be calculated. The results are represented in figures IVand V. In the first phase, the weight loss of the treatedsamples was 30-50 % of the untreated samples. Later, thedifference became even more pronounced : the weight loss ofthe treated samples was only 10 % in comparison with thetreated ones. This means that although there is an importantdifference between the treated and untreated samples, the useof a water repellent does not completely stop the action ofacid.

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3.3. Acid atmosphere

Samples were exposed to an atmosphere with a R.H. of 95 % andloaded with 0.6 % of SO 2 gas. A cycle of 7 hours of exposurefollowed by 17 hours of non exposure was repeated 28 times.After this period samples B showed a marked color-change withnoticeable corrosion and obvious crumbling of the surface.Samples DF showed some decay as well, although less dramatic.The untreated samples DO hardly showed any damage at all.Striking but in concordante with [3] is the fact that thetreated samples showed more damage than the untreated ones.The reason for this can be that, unlike acid rain, the SO 2 gasas well as the water vapor penetrates the stone. During thedrying period of the cycle, water as well as dissolved gas canevaporate from the untreated samples, while both are tapped inthe treated ones. As a consequence, contact of the stonematrix with acidity is much langer for treated than foruntreated samples.

3.4. Salt crystallization test

In order to establish the resistance of ferruginous sandstoneto salt crystallization and to control the effect ofconsolidants and water repellents on the crystallizationresistance, tests were carried out with sodium sulphate,sodium chloride and potassium nitrate solutions. Cubicsamples of 5 cm edge were placed with one side in a 0.345mole/1 solution of the three salts. For the treated samples,care was taken that the treated surface was faced upwards. Allspace between the samples and the side of the box wasthcroughly closed off with polystyrene in such a way that theevaporation of the water could only occur through the uppersurface of the samples. The boxes were placed in an oven at40 ° C for a period of 28 days. Every 7 days the samples wereremoved from the test boxes, the efflorescence washed off andthe damage recorded.In the case of Na 2SO4 the untreated samples withstood the testquite well, although some etching of the surface occurred.Exception has to be made for some soft samples of DO, whichwere further eroded. The samples treated with consolidants didnot suffer any appreciable damage, except for a reddishdiscoloration probably due to a reaction of the salts with theconsolidants. The samples treated with water repellents allshowed important damage. In most cases the hydrophobic layerwas completely exfoliated. Samples of type B seemed to resistslightly better than DO samples, although it is obvious thatthe occurrence of damage is rather a question of time than ofchemical/physical properties.In the case of NaCl none of the untreated samples suffered anydamage. On samples of type B the hydrophobic layer was neverexfoliated, but the surface showed a distinct etchingcomparable to the damage noticed on untreated samples exposedto the sulphate test. All surfaces of samples DF and DO werebadly damaged and several partially exfoliated. On allconsolidated surfaces only minimal dusting could be observed.With KNO 3 the control as well as the consolidated samples didnot show any damage. The hydrophobic layers were slightlyaltered, although no exfoliation was visible.

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3.5. Freeze-thaw test

To assess the durability of treated and untreated ferruginoussandstone samples were submitted to a freeze-thaw test. Thespecimens were placed in a box, lined with 4 cm of polystyreneand filled with wet sand. The samples were placed in such away that evaporation of water and frost attack could onlyoccur through the one exposed surface. The boxer were placedin a climatic chamber and exposed to a cycle of 15 hours -10 ° C followed by 9 hours of 15 ° C and 95 % R.H. After 42 cycleshalf of the samples as well as a series of new ones weresoaked for 8 hours in a 6.17 % solution of sodium sulfate.Then the frost-thaw test was continued for another 42 cycles.A visual inspection of the samples lead to the followingconclusions :- After the first 42 cycles, no important damage could berecorded for any of the samples, although some softer parts oftype DF and DO showed some erosion. This confirms our earlierfindings that the ferruginous sandstone is frost resistant.- After 84 cycles, the untreated samples not saturated withsulfates as well as the samples of type B saturated withsulfates resisted well. The softer parts of samples of type DFand DO saturated with salts deteriorated somewhat more. Insome cases samples treated with water repellents sufferedextensively. This was mainly the case for samples DF as wellas those samples where a deep impregnation (up to 20 mm) wasachieved. In some cases the stone under the hydrophobizedlayer was completely destroyed. Samples treated withconsolidant suffered more from the freeze-thaw test whensulfates were present. In contrast to the hydrophobizedsamples however there seemed to have beeen no major influenceof the impregnation depth and the consolidated layer neversplit off. Damage always occurred in the lower part of thestone where no consolidant was present.

3.6. Weather-OMeter ageing

Treated and untreated samples were exposed to an acceleratedweathering cycle in an Atlas Weather-OMeter XR35 for 813hours. The cycle consisted of :- 6 hours of exposure to - Xe-light (0.25W/m 2 /Nm at 340Nm)

- 45 ° C and 20 % R.H.- 6 hours of exposure to - Xe-light (0.25W/m 2 /Nm at 340Nm)

- 20 ° C and rain (spray of deionizedwater)

The WAC was measured every 168 hours after drying the samplestill constant weight at 60 ° C. Except for 2 samples on whichonly a small amount of water repellent was applied (less than120 g/m 2 ), none of the samples showed any water absorptionduring or after the test. Earlier tests [41 had proven thatunder these ageing conditions a lifetime of 15 - 20 years canbe expected.

4. REPLACEMENT

When the building material is so badly damaged that structuralproblems occur or that sculptured forms have lost all theirmeaning or even disfigure the monument, replacement has to beconsidered.

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It is obvious that as far as possible the same material as theoriginal should be used. As, however, the economie growth ofthe Netherlands in the 16th century did not affect the regionwhere ferruginous sandstone was traditionally used, a halt inconstruction activity occurred, resulting in a complete lossof the tradition to use this local building material. This wastrue to such an extend that, when restoration campaigns werelaunched again in the second half of the 19th century,sandstone from the Vosges was imported to replace the localironstone. This pink colored sandstone does not match theoriginal material and is still clearly distinguishable (fig.V). Even today the availability of ferruginous sandstonecauses problems and most contractors have to resort to usingsalvageable stone from demolished buildings. Only one sourceof restoration material is available nowadays : the sandpitsof Chaumont-Gistoux and Sart-Moulin.Larger quantities of Brusseliaan are available but the qualityis very inhomogeneous and variable and because of the moreeven structure and purple hue, due to higher manganesecontent, it remains distinguishable from the original Diest-stone.As the several quarries of this Diestiaan were closed in the16th century not because of lack of stone but because of lackof demand, it is quite certain that considerable quantities ofthis building material are still hidden in the region of NEBrabant. A thorough exploration of the region and afeasibility study for the reopening of old quarries isadvisable.

3. CONCLUSION

Ethyl silicate based products prove to be very effective forthe consolidation of ferruginous sandstone. Two to threeconsecutive applications result in a consumption of 3.5 to 51/m 2 , depending on the state of degradation of the stone andthe product used. Such treatment results in an importantincrease in abrasion resistance, without negative effects onthe drying speed. Hydrophobization can effectively be achievedwith 7 % solutions in white spirit of oligomericmethylsiloxanes. Consumptions between 0.20 and 1.7 1/m 2 can beexpected. Artificial ageing tests prove that thehydrophobization gives an excellent protection againstdegradation by lichen and acid rain, but not against acidatmosphere. Ferruginous sandstone containing soluble salts canbe consolidated but should never be treated with a waterrepellent, as this will induce the splitting off of theimpregnated layer or at least increase the surfacedegradation. For the replacement of structurally unstablestone it would be worthwhile to launch a exploration programfor the reopening of old quarries.

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REFERENCES

1. BOS, K. and DE WITTE, E. - Use and deterioration offerruginous sandstone in Northern Belgium. This edition.

2. NISHIURA, T and DE WITTE, E. - Drill-boring tests for thedetection of the deterioration and consolidation effect.Scientific Papers on Japanese Antiquities and Art Craft, Vol.30, 1985, pp. 11-14

3. SRAMEK, J. and PERINA, V. - Sensitive evaluation of theconservation efficiency of materials used for the treatment ofstone. Vth Int. Congress on Deterioration and Conservation ofStone, Lausanne 25-27.9.85, pp. 553-560

4. PIEN, A. - Technique de controle du traitement desmatériaux pierreux in situ. The Deterioration of BuildingMaterials, Ed. F. Auger, La Rochelle, 1990, pp. 69-77

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Table I : Consolidation of powders

Absorbed quantity of consolidant (weight %)

1 x 2x 3x 4x Total

B 23.7 7.5 7.8 2.6 45.2DF 27.6 8.4 11.9 2.5 50.4DO 27.3 5.6 7.0 4.3 44.2

Weight increase after complete hartlening (weight %)

1x 2x 3x 4x

B 9.3 12.7 13.6 15.7DF 9.5 12.4 14.3 15.6DO 9.7 10.9 12.8 13.8

B : non aged samples from the Brussels formationDF : non aged samples from the Diest formation,DO : naturally aged samples from the Diest formation

(collected from monuments)

Table II : Physical propertjes of untreated and completelyimpregnated samples

•111111~111111111111111•1111U.77

WP BD RD S DR

B 24.8 2796 2120 67 506B* 20.1 2717 2171 62 585

DF2 30.2 2723 1900 72 492DF2 * 24.2 2607 1977 71 653DF 3 27.9 1805 2021 73 300DF 3 * 22.5 2695 2088 70 578

* : treated samplesWP(%) Porosity accessible to water (RILEM I.1)BD (kg/m 3 ) : Bulk density (RILEM I.2)RD (kg/m3 ) : Real density (RILEM 1.2)S (%) : Saturation coefficient (RILEM II.1)DR : Drying Rate (1/m 2 .h)

112 2

Table III : Consumption of consolidants by naturally agedsamples

Absorbed quantity of consolidant/treatment (g/m 2 )

lx 2x 3x 4xTEGV DOx 1142 1140 1116 1229

DO 1149 1392 1394 1105YSOH DO 1354 1397 1395 1299x

DOY 1686 1689 1616 1781

Cumulative absorbed quantity of consolidant (g/m 2 )

TEGV DOxDODSOH OY

DO y

lx1142114913541686

2x2282254127513375

3x3398394041464991

4x4917504554456772

DO x Average value of a series of samples from 1 monumentDO : Average value of a series of samples from 1 monument,

other than Dx

Table IV : Consumption of water repellents in g/m 2

TEGOSIVIN HL100

ID : 2 mm 5 mm 10 mm

B 188 ± 103 241 ± 33 849 ± 182DF 316 ± 132 593 ± 79 1071 ± 312DO 242 ± 23 336 ± 30 606 ± 168

WACKER 090

ID : 2 mm 5 mm 10 mm

B 316 ± 97 303 ± 38 716 ± 137DE 444 ± 163 673 ± 127 1698 ± 312DO 256 ± 30 397 ± 73 707 ± 106

ID : impregnation depth

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<53 63 125 250 500 710 118053 90 180 355 600 1000 2000

Siev4

0.5 -

I- 0 400 800 12'00 1600 2000

ZCD 600 1000 1400 1800Hurrher of revoluflore

3.5

1 2-5 -

Tableafter

V : Water absorptiontreatment

BR

WAC 1

1.5 ml

WAC2*

WAC3

0

DF 1 (10') * 0

DF2 (4'10") * 0

DOx — 1 0

DO (3'50") 0 0

and drying rate before and

DR1 DR2 DR3 ID

480 * 66 7-20

320 * 45 10-20

420 * 82 60-70

364 234 60 4-7

441 472 191 5-20

WAC 1 :WAC2 :WAC 3 :

DR 1 :DR2 :DR3 :

ID :::

Water absorption before treatmentWater absorption after consolidationWater absorption after consolidation andhydrophobizationDrying rate (g/m 2 .h) before treatmentDrying rate (g/m 2 .h) after consolidationDrying rate (g/m 2 .h) after consolidation andhydrophobizationimpregnation depth of water repellentno measurement possible because of sandy surfacenot measured, as samples were not consolidated

Figure I : Granulometry ofpowders collected on monuments(naturally decayed stone)BR : Brussels formationDST Diest formation

Figure II : Abrasionresistance of untreated andtreated samples :A : D0 3 , untreatedB : D0 3 , consolidatedC : D04 , untreatedD : DO4, consolidated

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90

80

702e460E1,5c1

1. 4030

20

Of

DC oc

Figure III : Weight loss byacid solution (0 - 7 h)left : untreatedmiddle : Wacker 090right : Tegosivin HL700

Figure IV : Weight lossby acid sol. (7-200 h)left untreatedmiddle : Wacker 090right : HL100

Figure V : Diest, St. Sulpicius. The lighter parts arerestorations with sandstone from the Vosges.

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