CHOOSING DENTAL PLASTERS FOR USE IN THE CONSERVATION WORKSHOP
W. T. Chase, & Lynda A. Zycherman
ABSTRACT—Six dental plasters were tested and compared to determine their working properties and physical characteristics. The features examined were: viscosity, setting time, exotherm, hardness, smoothness, bubbliness, shrinkage/expansion, ink absorption, ease of engraving, and detail fidelity. The final choice of which plaster to use will depend on which properties are critical to the conservation task at hand. The results are given in tabular form.
One of the annoyances every conservator has to deal with is the cessation of manufacture of a reliable and familiar material. Recently, when our stock of Baker Speedrock Dental Stone was exhausted, to our dismay we found out that Engelhard Mineral and Mining no longer manufactures it or any substitute. We decided to test six other plasters, all commonly used in dental practice, to see if we could get a satisfactory replacement for Speedrock. The plasters we tested are listed in Table I.
TABLE I PLASTERS
Plaster test bars were cast in a rectangular open-face mold 14″ × 3″ × 1 ½″, constructed out of ¼″ plexiglas and self-adhesive tape. Using modelling clay (Kleen-Klay), we fastened a 12″ commercial ruler with gradations in relief to the base plate. We made changeable identification tags of Moyco Pink Base Plate Wax, engraved with the type of plaster, manufacturer, and date. These were stuck on to one corner of the base plate with a piece of Kerr 24 guage green casting wax, melted with a hot spatula. Before each pour, the mold was sprayed with Pattern Release 202 (National Engineering Products, 5454 Wisconsin Avenue, Washington, D. C. 20015).
Temperature during setting was measured with a stainless-steel sheathed chromel-alumel thermocouple, ⅛″d × 12″ long, penetrating vertically down into the center of the mold assembly. Temperature/elapsed time data for each run were recorded manually.
The plaster was mixed up in a rubber bowl, according to the proportions recommended by the manufacturer. The mix was vibrated with a Vibro-tool to eliminate bubbles. Plaster was spatulated or poured into the mold and the mold was vibrated for a few seconds to insure a faithful reproduction and smooth casting. After the plaster had set, the bar was removed, usually by disassembling the mold. The ruler was pried out of the plaster bar and reset on the mold baseplate; a new identification tag was fastened down and the mold reassembled for the next casting.
The finished plaster bars were tested in a rough and ready way. The features we looked at and the ranking system were as follows:
The plaster was set when a fingernail would no longer leave a mark. (Minutes)∗Exotherm
The maximum temperature reached and the time elapsed from pouring were noted. (Low/Medium/High °C/Minutes)∗Hardness
Measured on Mohs scale.∗Smoothness
Determined by running a finger over the set surface. (1 = Smoothest; 6 = Roughest)∗Bubbliness
Determined visually. (1 = Least; 6 = Most)∗Dimensional Change
The impression of the graded ruler was compared to the ruler. (1 = Least Dimensional Change; 6 = Most)∗Ink Absorption
A line was drawn on the plaster with a #00 rapidograph pen. The breadth of the dry lines were compared. (1 = Least; 6 = Most)∗Ease of Engraving
A deep line was made with an engraver's burin and the edges examined for irregularities. (1 = Best; 6 = Worst)∗Detail Fidelity
Comparison of impressions taken from the plaster bars with dental impression material. (1 = Best; 6 = Worst)
The results are set out in Table II.
TABLE II PROPERTIES OF PLASTER AS DETERMINED IN FGA LABORATORY June, 1981 (typed up June 26, 1981, WTC and LAZ)
It is clear that the choice of which plaster to use will depend on which properties are critical to the task at hand. Nevertheless, some of the properties shown, such as the Mohs hardness 6 of the Denstone and Stalite mixture, are quite surprising. The increased hardness, decreased ink absorption, and lower exotherm of the plasters mixed with Stalite are noteworthy, and tend to support the claims of the manufacturers. While our experiment could well be repeated with better controls on the experimental conditions and additional tests (breaking strength, tensile strength, etc.) and while the chemistry of the materials should be investigated, our tests have shown us that these dental materials form a group with a wide range of useful properties. The best replacement for Speedrock will probably be the Denstone with some Stalite added. We should also point out that the dental industry has many other materials which are worth investigating for conservation work.