CASTING DEFECTS

Dr. Sriharsha Vadapalli

DEFINITION

Casting is the process by which an object is formed by the solidification of a liquid that has been poured or injected into a mold.

Once the investment has set for an appropriate period of 1 hr for gypsum & phosphate bonded investment, it is ready for burn out

History

• 1905-LOST WAX PROCESS

• 1933-Replacement of CO-Cr for gold in RPD

• 1950-Development of resin veneers for gold alloys

• 1959-Porcelain fused to metal technique

• 1971-The Gold standard

• 1980-ALL-CERAMIC TECHNOLOGIES

• 1999-Gold alloy as alternative to Pd-Based alloys

LAWS OF CASTINGS -W.Patrick Naylor

• LAW I : attach the pattern sprue former to the

thickest part of wax pattern.• Pitfalls : Cold shuts, short margins, incomplete

castings

• LAW 2 : orient wax pattern so all the restoration

margins will face the trailing edge when the rim is positioned in the casting machine

• Pitfalls : cold shuts, short margin

• LAW 3 : position the wax pattern in a cold zone of

investment mold and reservoir in the heat centre at the casting ring

• Pitfalls : shrinkage porosity

• LAW 4 : a reservoir must have sufficient molten

alloy to accommodate the shrinkage that occurs within the restoration

• Pitfalls : shrinkage porosity and suckback porosity

• LAW 5 : do not cast a button if a connector

(runner)bar or other internal reservoir is used• Pit falls : shrinkage porosity, distortion ,

suckback porosity

• LAW 6 : turbulence must be minimised, if not

eliminated• Pitfalls : voids & surface pitting

• LAW 7 : select a casting ring of sufficient length

and diameter to accommodate the pattern to be invested

• Pitfalls : mould fracture, casting fins, shrinkage porosity

• LAW 8 : increase the wettability of wax pattern.

• Pit falls : bubbles.

• LAW 9 : weigh any bulk investment and

measure the investment liquid for a precise liquid : powder ratio

• Pitfalls : ill fitting castings

• LAW 10 : eliminate the incorporation of air in the casting

investment and remove the ammonia gas, the byproduct of phosphate bonded investments under vaccum.

• Pitfalls : small nodules, weak mould,distortion

• LAW 11 : allow the casting investment to set

completely before initiating the burn out procedure.

• Pitfalls : mould cracking or blow out , fins

• LAW 12 : use a wax eliminator (burn out) technique that

is specific for the type of pattern involved (wax/ plastic) and recommended for the particular type of casting alloy selected

• Pitfalls : cold shunts, short margins, cold welds, mold cracks , fins

• LAW 13 : adequate heat must be available to properly

melt and cast the alloy ( myers – 1936)• Pitfalls : cold shuts, cold margins, cold welds, rough

castings,, investment breakdown

• LAW 14 : while torch casting,use the reducing zone of

flame to melt the alloy and not the oxidising zone.

• Pitfalls : gas porosity , change in the coefficient of thermal expansion of alloy.

• LAW 15 : provide enough force to cause the liquid

alloy to flow into the heated mould.• Pitfalls : cold shuts, short margins, cold weld,

mould fracture, fins

• LAW 16 : cast turned towards the margin of wax

pattern .• Pitfalls : cold shuts, short margins, incomplete

castings

• LAW 17 : do not quench the ring immediately after

casting.• Pit falls : hot tears in the restoration.

DEFECTS IN CASTINGS

• DISTORTION

• SURFACE ROUGHNESS/ IRREGULARITIES

• POROSITY

• INCOMPLETE / MISSING DETAIL

DISTORTION

• CAUSES : By distortion of wax pattern by

carelessness of operator and by hardening of investment around the pattern .Also by uneven outward movement of proximal walls. The configuration of pattern,wax type and wax thickness influence the distortion

• EFFECTS : Not a serious problem except for the

unexplained inaccuracies in small castings.• PREVENTION : By proper manipulation of wax and

handling of pattern.There is probably not a great deal that can be done to control this phenomenon.

SURFACE ROUGHNESS, IRREGULARITIES, DISCOLORATION

• DEFINITION : Surface roughness is defined

as finely spaced surface imperfections whose height, width,and direction establish the predominant surface pattern

• Surface irregularities refer to isolated imperfections such as nodules that do not characterise the total surface area.

• Excessive roughness or irregularities on outer surface of casting necessitates additional finishing and polishing whereas irregularities on the cavity surface prevent a proper seating of an otherwise accurate casting.

• The greater surface roughness of casting compared to its wax pattern is related to the particle size of investment and its ability to reproduce wax pattern in microscopic detail

• This is not a major problem with proper manipulative technique.

Air bubbles

• Cause : Air bubbles form during or

subsequent to the investing procedure .They form nodules .

• Effect : Affect the fit of casting.• Prevention : Use Vacuum investing technique

• For Manual Method : Use mechanical mixer with Vibrations

before and after mixing. Use of Wetting Agent prevent air bubbles on

the surface of the pattern. Nodules not in critical areas can be

removed. Those in critical areas affect the fit of

casting.

Water films

• Cause : By seperation of investment from wax

pattern by slight movement of pattern. Also results when painting / wetting

agent does not result in an intimate contact of investment with the pattern.

Effect of High L : P Ratio

• Effect : Minute ridges or veins on the surface of

castings.• Prevention : Use of wetting agents.

RAPID HEATING RATES

EFFECTS : • Fins / Spines on castings.• It is the characteristic surface roughness seen

on rapidly heating the mould and high temp. of the molten alloy.

• Steam carry some salts used as modifiers into mould which are left as deposits on walls after the water evaporates.

• PREVENTION : Gradual Heating of the Mould over a time period of 60 min.

• Greater the Investment bulk , slower should be the rate of heating.

UNDER HEATING

EFFECT : • Voids / porosities formed when the

hot alloy comes in contact with carbonaceous residues .

• Tenacious carbon coating on casting is resistant to pickling.

• Common with low temp. investment technique.

LIQUID : POWDER RATIO

• EFFECT : Rougher casting by improper L: P Ratio.• PREVENTION : Gauge the Amount of Water and

Investment accurately.

PROLONGED HEATING

EFFECTS : • Common with High Heat Casting

Technique.• Roughened Walls of mould by

disintegration of investment.• Surface texture affected by sulfur

compounds which are the products of decomposition of the investment.( does not respond to pickling)

• PREVENTION : When the Thermal Expansion

Technique is employed - mould should be heated to casting temp. never higher than 700 deg.celsius and casting should be made immediately.

TEMPERATURE OF ALLOY

• CAUSES : Over Heating of Alloy.• EFFECTS : Surface roughness by disintegration of

investment surface.• PREVENTION : No Over Heating of alloy with Gas Air

torch.

CASTING PRESSURE

• EFFECT : Rough casting by raised pressure.• PREVENTION : With Air Pressure casting machine

( 0.10 – 0.14 Mpa gauge pressure)

Average centrifugal casting machine – 3 to 4 turns of spring

COMPOSITION OF INVESTMENT

• EFFECTS : Surface Roughness by coarse silica Surface texture affected by binder – quartz• PREVENTION : Follow ADA SP NO .2 for

investments.

FOREIGN BODIES

CAUSES : • Rough Crucible former with

investment clinging to it.• Carelessness in the removal of sprue

former.• Contamination from sulfur

• EFFECTS : Contamination result in surface roughness ,

incomplete areas or voids. Foreign particles involved produce sharp,

well defined deficiencies. Bright appearing concavities by flux carried

to the mold Surface Roughness Sulfur contamination – Black castings

IMPACT OF MOLTEN ALLOY

• CAUSES : Mis direction of Sprue former.• EFFECTS : Raised areas on casting that prevents its

seating.• PREVENTION : PROPER SPRUING to prevent Direct Impact of

molten metal at an angle of 90 degree to investment surface.

PATTERN POSITION

• CAUSES : CLOSE PACKING - pattern positioning in the same

plane.• EFFECTS : Breakdown / Cracking of investment.• PREVENTION : Keep 3 mm between patterns & place

the pattern in different planes.

CARBON INCLUSIONS

• CAUSE: Carbon absorbed by alloy during

casting from an improperly adjusted torch or carbon containing investment

• EFFECTS : Visible carbon inclusion and carbides

Wax Elimination & Heating

• For gypsum bonded investment – 500°C for hygroscopic technique.– 700°C for thermal expansion technique.

• Phosphate bonded– 700°C to 1030°C.

• Advisable to begin burn-out procedure when the mould is still wet.

Hygroscopic Low Heat Technique

• Obtains compensation expansion from 3 sources.– 37°C water bath expands wax pattern.– Warm water entering the investment mold from

top adds some hygroscopic expansion.– Thermal expansion at 500°C provides needed

thermal expansion.

Advantages of Low Heat Technique

– Less investment degradation.

– Cooler surface for smoother castings.

– Convenience of placing molds directly in 500°C

furnace.

• Care taken for sufficient burnout time.

• The molds should remain in furnace for atleast 60 min.

• Back pressure porosity great hazard in low heat

technique.

– Standardized hygroscopic technique was developed for alloys with high gold content; the newer noble alloy may require slightly more expansion. This added expansion may be obtained by making 1 or more of following changes.

1. Increasing water bath temperature to 40°C.2. Using two layers of liners.3. Increasing burnout temperature to a range of 600°C

to 650°C.

High-Heat Expansion Technique• Depend almost entirely on high-heat burnout to

obtain the required expansion, while at the same time eliminating the wax pattern.

• Additional expansion:– Slight heating of gypsum investments on setting.

Thus expanding the wax pattern.– Water entering from wet liner adds a small

amount of hygroscopic expansion to the normal setting expansion.

Gypsum Investments• Fragile and require use of metal ring.• Slowly heated to 600°C to 700°C in 60 mins. and held

for 15 to 30 min. at the upper temperature.• Rate of heating:– Smoothness.– Overall dimension of investment.– Too rapid heating.

• Cracking of investment outside layer expands more than center section.

• Radial cracks interior to outward.• Casting with fins or spines.

• Investment decomposition and alloy contamination is related to the chemical reaction between the residual carbon and sulfate binder.

• Calcium sulfate does not decompose unless heated above 1000°C.

• Reduction of calcium sulfate by carbon takes place rapidly above 700°C.– CaSO4 + 4C CaS + 4CO

– 3CaSO4 + CaS 4CaO + 4SO2

• This reaction occurs when gypsum investment are heated above 700°C in presence of carbon. SO2 as a product of this reaction contaminates gold castings and makes them extremely brittle.

• Sulfur gases are generated when investment is heated above 700°C.

• After casting temperature reached Casting should be made immediately.– Sulfur contamination.– Rough surface on casting.

• Improved investment formulations.– Gypsum investment with considerable amount of

cristobalite.

– Few investments may be directly placed into furnace at final burnout temperature

held for 30 min. & cast.

Factors affecting size and smoothness.

• Design of the furnace.

• Proximity of the mold to the heating element.

• Availability of air in the muffle.

Phosphate Investments• Obtain their expansion from following sources.– Expansion of the wax pattern.– Setting expansion (because of liquid used).– Thermal expansion

• Phosphate investments much harder and stronger than gypsum investments.

• Burnout temperature range from 750°C to 1030°C. • 315°C rapid heating held at the upper

temperatures for 30 mins.

• 2 stage heating:–Placed directly in furnace at top

temperature.–Held for 20-30 min. then cast

• Elimination of metal ring and liner to save more time.–Metal ring replaced with plastic ring i.e.

tapered.

Time allowable for casting

• Investment contracts thermally as it cools.• When thermal expansion / high heat

technique used.• Investment loses heat after heated ring

removed from furnace and mold contracts.

Low heat technique High heat technique

Temperature gradient not great.

Great temperature gradient.

Thermal expansion not important to shrinkage

compensation.

Thermal expansion important.

Burnout temperature on a fairly steep portion of

thermal expansion curve.

Burnout temperature on thermal expansion

plateau.

Casting Machines• Alloys are melted in one of the 4 following

ways:– In a separate crucible by a torch flame, and

cast centrifugally.– Electrically by a resistance heating or

induction furnace. Centrifugally by motor or spring action.–By induction heating. Centrifugally by motor

or spring action.–Vacuum arc melted. Cast by pressure in

argon atmosphere. • Molten metal may be cast by air pressure, by

vacuum, or both.

Torch Melting of Noble Alloy• Melted by placing on inner sidewall of crucible.• Fuel used - Natural or artificial gas and air.

- Oxygen air and acetylene.• Non-luminous brush flame with different combustion

zones should be obtained.–Mixing zone–Combustion zone.–Reducing zone.–Oxidizing zone.

• At proper casting temperature the molten alloy is light orange and tend to spin or follow the flame.

• Alloy should be approx. 38°C to 66°C above liquidus temperature.

• The casting should be made when proper temperature

is reached

• No more 30 secs should be allowed to elapse between

the time the ring is removed from the oven and the

molten alloy is centrifuged into the the mold

• When the alloy is molten,slide the crucible against the

ring,sprinkle flux over the metal

• Hold the casting arm so that the pin drops away

• Release the arm and rotate till it comes to rest

Use of flux for gold alloy

• It is desirable to use flux for gold alloy to minimize

porosity. To increase the fluidity of the metal film of flux

formed on surface of alloy prevents oxidation

• Reducing fluxes: Reduce oxides the present to free

metal and oxygen. Excellent in cleaning old alloy.

E.g. 1.Powdered charcoal

2.Fused Borax powder ground with Boric acid

powder

CENTRIFUGAL CASTING MACHINE

Torch melting centrifugal casting machine

Casting machine spring wound from 2 to 5 turns

Alloy is melted by a torch flame in a glazed ceramic crucible attached to the “broken-arm” of casting

machine.

Broken arm feature accelerates the initial rotational speed of the

crucible and casting ring.

Torch flame is generated from a gas mixture of propane and

air.

Machine is released and spring triggers the rotation motion.

As the metal fills the mold, a hydrostatic pressure gradient develops along the length of

the casting.

• The pressure gradient from the tip of the casting to the bottom surface is quite, sharp and parabolic in form, reaching zero at the button surface.

• Pressure gradient before solidification reaches 0.21 to 0.28MPa (30 to 40 psi) at the tip of casting.

• Greatest rate of heat transfer to the mold is at the high pressure end of the gradient (tip of the casting).

SPRING-WOUND CASTING MACHINE WITH ELECTRIC RESISTANCE FURNACE

Electrical Resistance-Heated Casting Machine

• Current is passed through a resistance heating conductor, and automatic melting of the alloy occurs in a graphite or ceramic crucible.

• Advantages:– For metal ceramic prosthesis.– Base metals in trace amounts that tend to oxidize

on overheating.– Crucible located flush against casting ring.

• Carbon crucibles should not be used in melting of:– High Pd– Pd-Ag– Ni-Cr– Co-Cr

INDUCTION MELTING CASTING MACHINES

• WATER COOLED INDUCTON COIL

• VERTICAL CRUCIBLE POSITOINED WITHIN INDUCTION COIL

Induction Melting Machine• Alloy is melted by induction field that develops within a

crucible surrounded by water cooled metal tubing.

• The electrical induction furnace is a transformer in which an

alternating current flows through the primary winding coil

and generates a variable magnetic field in the location of the

alloy to be melted in crucible.

• Once the alloy reaches casting temperature in air/vacuum it is

forced into mold by centrifugal force by air pressure, or by

vacuum.

• More commonly used for melting base metal alloys.

Direct Current Arc Melting Machine

• Direct current arc is produced between two electrodes- the alloy and water cooled tungsten electrode.

• The temperature between the arc exceeds 4000°C.

• Has high risk of overheating.

Vacuum or Pressure Assisted Casting Machine

• Molten alloy is heated to casting temperature drawn into the evacuated mold by gravity or vacuum and subjected to additional pressure to force the alloy into the mold.

• Used for titanium and titanium alloys. Under vacuum arc heated-argon pressure casting machine.

Casting Crucibles

• Clay.• Carbon.• Quartz.• Zirconia-alumina.

Cleaning the casting

• Quenching: ring is removed and quenched in water as soon as the button exhibits a dull red glow

• Purpose of quenching • All the intermediate phases are changed to

a disorderly state and ductility is increased• Soft granular investment that is easily

removed

Devesting or Sand blasting

• Casting is held in an sand blasting machine to clean the investment from the surface. The blasting materials used are:– Sand shells– Recycled aluminium oxide with pressure of 100psi– Garnet

PicklingRemoval of oxide residues of carbon by heating

the discoloured casting in an acidAdvantages of HCl:

Aids in removal of residual investment as well as oxide coating

Disadvantages:Likely to corrode laboratory metal furnishingsFumes are health hazard

Method of cleaning Place the casting in test tube or dish and pour

acid over it Other methods: Heating the casting and then

dropping into the pickling solution• Finishing and polishing : Brown or White AlO2

stones are used• Cleaning : Using a hemostat, steam or clean

framework

CASTING DEFECTS• Casting is defined as the act of forming an

object in a mold. The object formed from this procedure is called a casting.

• Any impressions or irregularities that result in unsuccessful casting which interfere with the fit of the final restoration-basically classified into 4 categories–DISTORTION – SURFACE ROUGHNESS AND IRREGULARITIES–POROSITY – INCOMPLETE OR MISSING DETAIL

POROSITY• Defined as the state of quality of having

minute pores,openings or interstices.• EXTERNAL POROSITY : It can be a factor in surface

roughness or manifestation of internal porosity.

• INTERNAL POROSITY : It can weaken the casting and it extends

to surface, lead to discoloration.In severe cases, it causes leakage at tooth – restoration interface resulting in secondary caries.

TYPES OF POROSITIES

• SOLIDIFICATION DEFECTS : A) localised shrinkage porosityB) Microporosity• TRAPPED GASES : A)Pin hole porosityB)Gas inclusionsC)Sub surface porosity• RESIDUAL AIR

SURFACE IRREGULARITIES• Air bubbles• Water films• Rapid heating under heating and prolonged

heating• Liquid/powder ratio• Temperature of alloy• Casting pressure • Composition of investment• Foreign bodies• Impact of molten alloy• Pattern position• Carbon inclusions

SURFACE IRREGULARITIES

A)AIR BUBBLES

B)WATER FILMS C)FOREIGN BODY

BLACK COATED NOBLE METAL ALLOY

POROSITY• SOLIDIFICATION DEFECTS

• LOCALISED SHRINKAGE POROSITY• MICROPOROSITY

• TRAPPED GASES• PINHOLE POROSITY• GASINCLUSIONS • SUBSURFACE POROSITY

• RESIDUAL AIR

LOCALISED SHRINKAGE POROSITY

• Premature termination of flow of molten metal

• Sprue freezes before feeding complete to casting proper

• Retain heat because of bulk and heat center of ring

• Small chill set sprues

LOCALISED SHRINKAGE POROSITY• CAUSES : Incomplete feeding of molten metal

during solidification. commonly it occurs near to sprue – casting junction or in the interior of crown near the sprue

• EFFECTS : A Localised shrinkage void in the last

portion of casting that solidifies• PREVENTION : Attach one or more small gauge

sprues (18 gauge) at the the surface most distant from the main sprue attachment.

MICROPOROSITY

• Occurs by solidification shrinkage in fine grain castings

• Occur from rapid solidification if mold /casting temperature is too low

MICROPOROSITY

• Seen in fine grain Alloy castings when solidification is too rapid for the microvoids to segregate to liquid pool.

• It can occur when mould or casting temp. is too low.

• EFFECTS : Small irregular voids, not a serious defect.

SUCK BACK POROSITY

• Occlusoaxial/incisoaxial line angle

• Hotspot • Flaring the point of

sprue attachment• Lowering casting

temperature by 30 degrees

SUCKBACK POROSITY• A Type of Localised Shrinkage Porosity

occuring in the interior of a crown near the area of sprue.It forms when a Hot Spot has been created by the hot metal impinging from the sprue channel on a point of mould wall.

• It causes the local region to freeze last.• Can be eliminated by Y – sprue instead of

single sprue , flaring the point of sprue attachment and by reducing the mould – melt temperature differential.

PINHOLE POROSITY AND GAS INCLUSIONS

• Entrapment of air during solidification• Spherical contour• large spherical porosities caused by gas

occluded by a poorly adjusted torch flame

SUB SURFACE POROSITY

• CAUSES : Not completely established.Caused by

simultaneous nucleation of solid grains and gas bubbles at the first moment that the metal freezes at the mould walls.

• PREVENTION : Control the rate at which the molten

metal enters the mould.

SUB SURFACE POROSITY

• simultaneous nucleation of solid grains and gas bubbles at the first moment that the alloy freezes at the mold walls

• rate of molten metal that enters the mold

BACK PRESSURE POROSITY

• Large concave depressions

• Inability of air in mold to escape through the pores in the investment

• Proper burnout,adequate mold and casting temperature,high casting pressure,proper l/p ratio

INCOMPLETE CASTING • MOLTEN ALLOY PREVENTED FROM COMPLETELY

FILLING MOLD• Insufficient venting and high viscosity• Incomplete elimination of wax residues• Shiny margins caused by CO left by residual wax• Temperature of alloy should be raised so that its

viscosity and surface tension are lowered.

INCOMPLETE CASTINGSCAUSES : • Insufficient venting of mould• high viscosity of fused metal• Insufficient venting is related to the

backpressure exerted by air in mould and incomplete elimination of wax residues from mould.

PREVENTION : • Apply appropriate casting pressure for 4 sec .

• Raise the Alloy Temperature greater than its Liquidus temperature so that its viscosity and surface tension are lowered and it does not solidify prematurely as it enters the mould.

INCOMPLETE CASTING

ROUNDED INCOMPLETE MARGNS

RESIDUAL AIR

• ENTRAPPED AIR POROSITY / BACK PRESSURE POROSITY :

• CAUSES : Inability of air in the mould to escape

through the pores in investment or by pressure gradient that displaces the air pocket towards the end of investment via molten sprue and button.

• Its incidence is increased by dense modern investments , increase in mould density caused by Vacuum investing and tendency for mould to clog with residual carbon when low heat technique is used .

• EFFECTS : Large concave Depressions• PREVENTION : Proper burn out , adequate mould &

casting temp. , sufficiently high casting pressure, proper l: p ratio.

TRAPPED GASES

PINHOLE - GAS INCLUSION POROSITY: • Both are similar except for their

size, gas inclusion porosities are LARGER.CAUSES : • Due to Gas Entrapment during

Solidification.• Copper & Silver can dissolve oxygen while

pt & pd have a strong affinity for H2.

• EFFECTS : In the form of voids or Discoloration• PREVENTION : Premelting Gold Alloy on a Graphite

Crucible or Graphite Block

Summary

• The wax pattern is the precursor of the finished cast restoration. As it will be duplicated exactly through investing and casting, the final restoration can be no better than its wax pattern. A successful result depends on careful handling of the wax.

• Strict adherence to standardized procedures ensure a good and long lasting cast restoration obliterating for costly remakes and annoyance to the patient and the operator.

REFERENCES• Anusavice “Skinners science of dental materials”. Tenth Edition.• Asgar K., Arfaei A.H. “Castability of crown and bridge alloys”. J

Prosthet Dent 1985; 54: 60-63.• Donovan T.E., White T.E. “Evaluation of an improved centrifugal

casting machine”. J Prosthet Dent 1985; 53: 609-612.• Eames B.W. “Evaluation of casting machines for ability to cast sharp

margins”. J Oper Dent 1978; 3: 137-141.• Hamaka H., Doi H., Yoneyama T., Okuno O. “Dental casting of

titanium and Ni-Ti alloys for a new casting machine”. J Dent Res 1989; 68: 1529-1533.

• Hruska A. “A novel method of vacuum casting titanium”. Int J Prosthodont 1990; 3: 142-145.

• Laboratory manual of the requirements used for casting – BEGO, DENTAURUM, OROTIG,MEGAPULS 3000.

• Michael J. Lessiter, Ezra L. Kotzin, Timeline Summer 2002,www.castsolutions.com.

• Earnshaw R. ‘The effects of additives on the thermal behaviour of gypsum bonded casting investments part I. Aus. Dent. J. 20 : pp27, 1975.

• Teteruck W.R. and Mumford G. ‘The fit of certain dental casting alloys using different investing materials and techniques’. J. Prosthet. Dent. 16 : pp910, 1966.

• J. F. Jelenko Co: Thermotrol Technician, 34 : No.1, Winter, 1980. • Neiman R. and Sarma A.C. : ‘Setting and thermal reactions of phosphate

cements’. J. Dent. Res. 9 : pp1478, 1980.• Allen F.C. and Asgar K : ‘Reaction of cobalt chromium casting alloy with

investments’. J. Dent. Res. 45 : pp1516, 1966.• Dootz E.R., Craig R.C. and Peyton F.A. : ‘Simplification of chrome-cobalt

partial denture casting procedure’. J. Prosthet. Dent. 17 : pp464, 1967.• Mabie C.P. : ‘Petrographic study of the refractory performance of high-

fusing dental alloy investments: II silica-bonded investments. J. Dent. Res. 52 : pp758, 1973.

• Mahler P.B. and Ady A.B. ‘An explanation for the hygroscopic setting expansion of dental gypsum products’. J. Dent. Res. 39: pp 578, 1960

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