Effect of burnout temperature on strength

of gypsum-bonded investments

W.K. Luka, B.W. Darvellb,*

aDental Technology, Faculty of Dentistry, Prince Philip Dental Hospital, The University of Hong Kong, 34 Hospital Road,

Hong Kong, People’s Republic of ChinabDental Materials Science, Faculty of Dentistry, Prince Philip Dental Hospital, The University of Hong Kong, 34 Hospital Road,

Hong Kong, People’s Republic of China

Received 1 February 2002; accepted 29 May 2002

Abstract

Objective. To investigate the variation of the strength of gypsum-bonded dental investments with burnout temperature.

Methods. The disc-rupture test was employed at burnout temperatures ranging from 450 to 800 8C for four products (Beauty Cast,

Cristobalite, Novocast (all WhipMix) and Deguvest California (Degussa)). In this test, an investment disc is created inside an investment

mold such that it forms a diaphragm across the mold space, being an integral part of the mold. Copper was cast into molds of this type, and

whether the disc had ruptured or not was determined by inspection when cold. The amount of copper cast in successive tests was varied in

staircase fashion up and down depending on whether the disc survived or failed. The strength of the investment was represented by the 50%

point of the transition from survival to rupture.

Results. The strength of gypsum-bonded investment is temperature sensitive, there being marked differences between the products tested.

Overall, the strength ranking (temperature range average/MPa) was: Cristobalite (10.1) . Beauty Cast (7.8) . Novocast (5.1) . Deguvest

California (3.4). The strength ranking at high temperatures differs from that known for the room temperature values.

Significance. The use by manufacturers of room-temperature strength data conveys no information about high temperature behavior.

Investment properties should be optimized by reference to behavior under casting conditions.

q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved.

Keywords: Burnout temperature; Gypsum-bonded

1. Introduction

Dental casting was first introduced by Philbrook in 1896

[1] and was popularized by Taggart [2]. Early investments

consisted mainly of a mixture of powdered silica and plaster

of Paris [3,4], what is now called gypsum-bonded invest-

ment (GBI). Apart from those major constituents, modifying

agents such as sodium chloride and boric acid, pigments and

reducing agents may be added to adjust the properties. The

strength of an investment is important both for handling the

mold at room temperature and to resist thermal stresses on

heating and casting forces at high temperature, although

strength under the latter conditions has rarely been

investigated. Gypsum- and phosphate-bonded investments

are the two most common types currently in use in dentistry.

Luk [5] and Luk and Darvell [6–8] have reviewed and

studied the high-temperature strength of phosphate-bonded

investments (PBI). However, very little has been reported

on the high-temperature strength of GBI, although such

information would be worthwhile in understanding the

processes involved but also with a view to guiding users in

their selection of products, as well as to specifying

comparative or standards compliance tests.

Coleman [4] performed high temperature axial com-

pression tests on GBI cylinders, the specimens being tested

inside a furnace. It was observed that strength changed with

temperature, but there was no clear trend. Also noted was a

marked decrease in strength if the heated investment was

allowed to cool to room temperature before performing the

test. This was explained as due to cracking caused by too

rapid cooling, but although this is of interest for devesting it

is of limited practical importance. Hunter [9] measured

compressive strength before and after heating to 700 8C,

Dental Materials 19 (2003) 552–557

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0109-5641/03/$ - see front matter q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0109-5641(02)00104-5

* Corresponding author. Tel.: þ852-2859-0303; fax: þ852-2548-9464.

E-mail address: b.w.darvell@hku.hk (B.W. Darvell).

when the test was done immediately after removal of the

specimen from the furnace. The various brands of invest-

ment had different strengths after heating, but clearly

removal of the specimen from the furnace could have

caused some cracking as it cooled and thus have lowered the

observed strength. Dootz et al. [10] measured the compres-

sive strength of denture casting investments at room

temperature and at their usual burnout temperature in a

furnace and reported simply that strength either increased or

decreased at high temperature. Earnshaw [11] found that the

compression strength at 700 8C was increased with boric

acid and decreased with sodium chloride, compared with the

values at 21 8C, these being additives used to reduce

gypsum shrinkage. Ohta et al. [12] found that the

compressive strength of two GBIs tended to decline when

the temperature was increased, said to be greatly affected by

the thermal transformation of the silica, and decreased

further during cooling. Ohno et al. [13] reported that the

‘state of the calcium sulfates is the main factor influencing

variations in the compressive strength at a particular

temperature’, which recognizes decomposition of the binder

as being relevant. However, Chew et al. [14] found that the

compressive strength at 700 8C was higher than that

measured at room temperature for setting time 2, 6 and

12 h although no explanation for this was advanced. For the

24 h setting time, the strength at 700 8C was the same as that

at room temperature, but it is not clear whether this was

understood to be by chance. As can be seen from these

reports, there are inadequacies and inconsistencies in need

of further study.

Some basic results are well-known. Porosity decreases

strength, so strength is greater when the investment is

mixed under reduced pressure (to remove air bubbles),

when less water is used for mixing (to reduce the volume

fraction of porosity), and when it sets under pressure

(which reduces the size of air bubbles) [15–18]. Not

surprisingly, compressive strength does vary with time: for

example, Osborne and Skinner [19] found, for six GBIs,

that it was greater at 7 d than at 1 h for a range of water–

powder ratios, while Finger et al. [20] showed that this

was true for 24 h vs. 2 h for three GBIs. This may in part

be due to the cementing action of solutes after evaporation

of excess water. The so-called ‘hygroscopic’ expansion

technique, introduced by Scheu [21], can reduce strength

[22], an effect which may be explained by the associated

increase in the volume fraction of porosity. However, all

of these strengths were measured at room temperature,

and this, unfortunately, has little bearing on the high-

temperature strength. Furthermore, since the situation

under casting conditions, with possibly abrupt loading

and thermal shock from the molten metal, is not

represented, it is not possible to relate such data to actual

service behavior.

On heating, the strength of GBI can be affected by the

thermal transformation of silica [12,23], thermal decompo-

sition of the binder [13,24,25] and high-temperature ‘bond’

[26–28] or ‘sinter bonding’ [29,30]. Since these processes

must occur during the ordinary burnout process it is to be

expected that strength varies according to each type of event

over the corresponding temperature ranges, that is, in

addition to more fundamental differences relating to the

constitution and other characteristics of the as-set binder. It

therefore follows that investment strength should vary with

burnout temperature, as has been shown clearly in the case

of PBI [7].

According to Weast [31], dehydration of calcium

sulphate dihydrate takes place at 128 8C to give the

hemihydrate, which has then lost its water by 163 8C to

give anhydrite, of which there are three polymorphic forms.

The low temperature III-CaSO4 (hexagonal) starts to

transform to II-CaSO4 (orthorhombic) above 200 8C [31]:

which in turn transforms to I-CaSO4 (cubic) at about

1200 8C [32]. The decomposition reactions result in major

disruption of the microstructure, but whether the anhydrite

is well-crystallized or not, its transformations must be

associated with interparticle stresses and displacements as a

consequence of the lattice parameter changes.

The effect of dehydration on the compressive strength

of GBI has been demonstrated by Ohno et al. [13]:

drying caused a sharp rise in strength from 100 to 175 8C

(internal temperature ranged from 60 to 100 8C),

dehydration from dihydrate to hemihydrate caused a

sharp drop in strength from 175 to 225 8C (internal:

100–160 8C), dehydration from hemihydrate to anhydrite

(III-CaSO4) caused an increase in strength to 300 8C; and

transformation from III-CaSO4 to II-CaSO4 caused a

slight decrease in strength to 450 8C. Apart from

indicating clearly that strengths before and after burnout

can be expected to differ, behavior in the usual range of

burnout temperatures was unaddressed. According to

Anderson [23], dehydration of binder from 100 to 125 8C

and transformation of the silica were the two ranges of

weakness of gypsum investments.

The temperature at which the calcium sulphate itself

starts to decompose is controversial. O’Brien and Nielsen

[33] found that calcium sulphate alone will not decompose

until 1200 8C, while Jones [27] reported extremely rapid

decomposition between 900 and 1000 8C but suggested that

the process might start as low as 650 8C. Matsuya and

Yamane [34] reported that decomposition began at about

900 8C and led to the formation of CaSiO3 as a major

product and Ca2SiO4 as a minor one below 1250 8C. The

reaction is [33]:

CaSO4 þ SiO2 ! CaSiO3 þ SO2 þ12

O2 ð1Þ

The decomposition of calcium sulphate, therefore, does not

seem to take place in the normal range of manufacturers’

recommended burnout temperatures for GBIs, roughly

500–750 8C. However, it has been suggested that these

materials should not be heated above 700 8C because of a

reaction with any carbon residue which generates

W.K. Luk, B.W. Darvell / Dental Materials 19 (2003) 552–557 553

sulphur dioxide:

CaSO4 þ 4C ! CaS þ 4CO ð2Þ

3CaSO4 þ CaS ! 4CaO þ 4SO2 ð3Þ

Sulphur dioxide contaminates gold castings in particular,

making them brittle [33]. These reactions also lead to the

breakdown of the mold surface and increased casting

roughness.

Axial compression, direct tension, indirect tension and

three-point bending are common mechanical strength tests.

Direct tension is not suitable for brittle materials such as

dental stone and investment because of alignment problems.

In dentistry, axial compression (‘compressive strength’) [4,

11,35] and three-point bending [36,37] are the two most

common strength tests employed for investments. However,

the mode of failure of the former is not well understood,

although it can be said that the material being tested does

not fail in compression [38]. The mode of failure under the

latter test is due to tension [39]. Both tests are difficult to

perform at high temperatures, given the normal time scale of

burnout. It is presumed that the actual detailed state of the

material depends on the full time-at-temperature profile

because many processes are kinetically limited, diffusive

ones in particular. In addition, such tests do not take into

account the effect of the heat of the casting metal, nor mimic

the changing of rate of loading as in an actual casting

process. To answer such difficulties, Luk and Darvell [6]

introduced the disc-rupture test, which is conducted through

an actual burnout and casting process such that those factors

and others are automatically taken into account. The mode

of failure in this test is clearly in tension [40]. It is believed

that the strength obtained by this method is representative of

that of actual service.

Given the lack of information about the actual high-

temperature strength, it was the purpose of the present work

to investigate the variation of strength of GBI with burnout

temperature.

2. Materials and methods

The disc-rupture test described by Luk and Darvell [6]

was employed with minor modifications. The spacer

thickness was fixed at 1.4 mm, while different products

were used for the surfactant (Aurofilm, Bego, Bremen,

Germany) and ring liner (Kera-Vlies, Dentaurum, Pforz-

heim, Germany). Four products were tested (Table 1). Each

investment was first hand-mixed with distilled water for

about 15 s to allow wetting of the dry powders and then

mechanically mixed (Combination Unit, Whip Mix, Louis-

ville, KY, USA) under reduced pressure (8 kPa) for 30 s. The

material was allowed to bench set for 30 min after investing

the pattern. Burnout started from room temperature (about

23 8C), with 8 K min21 ramp to the final temperature, which

ranged from 450 to 800 8C in 50 8C steps (apart from testing

Cristobalite at 770 8C instead of 750 8C due to an error), and

held as necessary for a total heating time of 100 min. The

casting process was initiated promptly at the completion of

that burnout. An induction centrifugal casting machine

(Fusus NG1, Galoni, Italy) (R ¼ 0.185 m, n ¼ 7.33 r s21;

[6] for strength calculation), carbon crucible, and copper at

1100 8C (representing a typical gold alloy casting tempera-

ture) as the casting metal were used for all trials. The trials

conducted are summarized in Table 2. The method of

determination of the transition point at each temperature

followed that described by Luk and Darvell [7].

3. Results

The results are shown in Fig. 1. The strength of each

investment varied with burnout temperature, but in a variety

of ways. Beauty Cast and Cristobalite showed the greatest

variation and exhibited two strength peaks, at ,550 and

,700 8C. Deguvest California showed a minimum around

600–650 8C, while the others showed their lowest strength

at 800 8C (the upper limit tested). Overall, Cristobalite had

Table 1

Details of the gypsum-bonded investments tested

Beauty Cast Cristobalite Novocast Deguvest California

Water-powder ratio (ml g21) 30:100 30:100 35:100 36:100

Mixing time (s) 30 30 30 30

Bench set (min) 30 30 30 30

Burnout temp. (8C) 515a, 650b 650 650 750 max.

Compressive strength, wet (MPa) 5 5 7 (not reported)

Colour Reddish brown White Grey White

Batch no. 092599001 9/7 041600002 043100001 200781

032598002 4/7

Silica form Quartz High cristobalite Quartz and cristobalite (unknown)

Manufacturer WhipMix c WhipMix WhipMix Degussad

a Max. for hygroscopic technique.b Thermal expansion technique.c Louisville, KY, USA.d Hanau, Germany.

W.K. Luk, B.W. Darvell / Dental Materials 19 (2003) 552–557554

the highest strength (the average over the temperature range

was ,10.1 MPa) followed by Beauty Cast (,7.8 MPa),

then Novocast (,5.1 MPa), with Deguvest California

(,3.4 MPa) weakest.

4. Discussion

The present results certainly indicate that the strength of

GBI is markedly temperature-dependent, but the lack of

commonality of behavior makes explanation problematic.

There is yet room for the study of the composition, structure

and behavior relationships of the system.

Room-temperature strength (wet strength) for the Whip-

Mix products are in the ratios 5:5:7 (Table 1), while for the

temperature range 450– 800 8C the mean values are

approximately in the ratios 10:8:5, for Cristobalite, Beauty

Cast and Novocast, respectively. Clearly, one is not

predictive of the other, as Taylor et al. [41] have already

pointed out. This lack of correlation has also been observed

for PBIs [7]. Thus, the high-temperature strength of GBI

may be affected by any changes in the binder (dehydration,

transformation, decomposition) and refractory (displacive

Fig. 1. Variation with temperature of the strength of the gypsum-bonded investements tested. Trials resulting in disc rupture indicated by the solid symbol, in

survival by the open symbol. Estimated transition points marked ‘ þ ’ and joined by spline curve for eye-guidance only.

Table 2

Number of test castings for each product and temperature tested

Temp (8C) Beauty Cast Cristobalite Deguvest Novocast

450 10 14 18 11

500 15 12 12 12

550 9 15 22 14

600 16 10 18 15

650 15 14 12 15

700 14 25 12 17

750 11 15a 11 15

800 12 20 12 16

a At 770 8C.

W.K. Luk, B.W. Darvell / Dental Materials 19 (2003) 552–557 555

transformation). In addition, sintering and reactions

between components may occur.

The temperature range of the present test, from 450 to

800 8C, encompasses the normal range of burnout tempera-

tures for GBIs for dental casting. Extending these tests to

temperatures below 450 8C would require a much longer

time for wax elimination and therefore did not fit the present

burnout schedule. But since any changes that have occurred

at a lower temperature could affect the strength at higher

temperatures, it is necessary to consider existing infor-

mation on the changes in GBI below 450 8C including

dehydration, transformation, and decomposition of binder.

These have been discussed earlier and other factors will now

be addressed.

4.1. Silica transformation

Quartz and cristobalite are the two allotropic forms of

silica that are commonly employed for the formulation of

dental investments. There is an a-to b-phase transformation

for cristobalite and quartz from 220 to 275 8C and at 573 8C,

respectively, [42] and these lead to volumetric expansion on

heating. Such expansion has been utilized to compensate for

much of the alloy thermal contraction, but it must lead to a

slightly less dense mass which may therefore weaken it. In

the case of cristobalite, the expansion is isotropic and the

change is rapid, this would create a differential shearing

movement between the refractory material and the binder,

creating stress and therefore potentially crack nucleation

sites. In the case of quartz the expansion is anisotropic and

may exaggerate the differential shear and thus the localized

cracking and disruption of contact points within the mass.

Ohta et al. [12] found that the high-temperature

compressive strength was greatly decreased by thermal

transformation of crystallization of silica. This agrees with

Anderson [23] but not Ohno et al. [13] who indicated that

there was only a slight effect for cristobalite but none for

quartz. In the present study, the two investments that

contained quartz, Beauty Cast and Novocast, showed a

decrease in strength from 550 to 600 8C, but the tendency

was very weak for the Novocast, if present. If the

cristobalite investments are affected, it cannot be demon-

strated by the present data as the transformation lies below

the test temperature range.

4.2. Thermal healing

During the burnout process, cracks occur in the

investment mass because of thermal stresses. These arise

from uneven heating and variation in the expansion on

heating between components as well as effects due to

decomposition reactions. Jones [37] claimed that ‘thermal

healing’ occurred if small proportions of a liquid phase were

present and attributed increase in strength at some

temperatures to this effect. By thermal healing was meant

the ‘repair’ of microcracks; the mechanism is obscure.

Diffusive processes may allow sintering or crack blunting to

occur, which would increase strength, but the involvement

of a liquid phase cannot, on the face of it, be beneficial. The

complexity of the present results suggests that several

factors may be operating, depending on composition.

4.3. Casting temperature

Luk and Darvell [8] indicated the importance of

accounting for the effect of metal temperature during

casting. They found that higher casting temperatures could

effectively reduce the strength of PBIs by raising the

investment temperature.

The temperature difference between the casting alloy and

the investment ranged from 300 to 650 8C in the present

tests. At the moment when the casting alloy reaches the

investment, a thermal shock is experienced. This might

cause melting or immediate cracking. Such melting might

occur for investments containing sodium chloride (m.p.

801 8C), for example. The melting temperatures of sodium

salts are normally low, e.g. sodium sulphate (m.p. 884 8C),

sodium disilicate (m.p. 874 8C) [31]. In addition, reaction of

investment constituents with oxides from the casting alloy

may also cause low-melting compounds or mixtures such as

eutectics to form. All of these possible events can reduce

the strength of the investment. It is to be expected that the

strength is lower if the casting temperature is higher than

the mold temperature, as is usual, and has been demon-

strated [8].

The surfaces of the castings in contact with the

investment disc for survival cases were not flat but usually

curved, a phenomenon that was also noted for PBIs [8].

This indicates that the investment disc deformed plastically

before fracture, presumably due to partial melting and

softening occurring when the molten metal came into

contact with the investment disc. Thus, investment discs

may have failed not only due to the casting force but also

due to the extra heat from the casting alloy which induced

melting and softening at the investment. Luk [5] found that

the breakage of the investment disc could occur between

0.3 and 5.7 s from start of the casting action but that the

cast metal had already been delivered into the mold in

about 0.17 s, and that the casting machine had already

achieved its maximum speed in about 0.3 s. The delay in

failure of the investment disc indicates that there must be

something besides the casting force that caused the disc

failure.

The plastic deformation of investment molds during

casting is potentially a serious problem that has hitherto

gone unrecognized, although the behavior has been noticed

before [37]. Much emphasis is placed on the dimensional

accuracy of castings as affected by the (regular) expansions

and contractions of impression, model, pattern, mold and

metal. But while the effects of constraints on these changes

(such as from a casting ring) have been considered, plastic

deformation of the mold itself during casting has not. This is

W.K. Luk, B.W. Darvell / Dental Materials 19 (2003) 552–557556

clearly a potential contributor to the fitting inaccuracy of

cast devices, although its effects would not be easily visible

or detected without specific effort. Perhaps this needs to be

investigated.

It is clear that the strength of GBI is temperature-

sensitive and that the strength at room temperature cannot

represent that at a higher temperature. To explain the

variations at least requires knowledge of the composition of

the materials in detail, although for commercial reasons this

is not likely. Nevertheless, it is plain that the present

approach could be used to study such effects, enabling the

more-precise tailoring of high temperature properties,

possibly overcoming the plastic deformation if this is

confirmed to be an issue. As it stands, the establishment of

high-temperature strength criteria for product certification is

now seen to be entirely feasible in a service-relevant

manner.

Acknowledgements

We thank the Whip Mix Corporation and Degussa A.G.

for their generous donation of the investments used in this

work.

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