c. Setting Time. The initial setting time for plaster of Paris is 5 to 10 minutes.
In this stage, the plaster loses its glossy appearance and is hard enough to hold for
carving. The final setting time is approximately 45 minutes. In this stage, the plaster
achieves a dry, hard condition. The setting of plaster can be hastened by using less
water, by mixing longer, by using chemical accelerators, or by using warm water (up to
85 F (29 C)). Reversing these processes or using chemical retarders lengthens the
setting time. The most satisfactory results will be obtained by following the
a. Uses. Artificial stone is used in making master casts and dies and for general
laboratory use when a very hard, strong product is needed. Artificial stone particles are
nonporous. Therefore, the finished product is hard and dense. This provides an
excellent master cast for the fabrication of prosthetic restorations. The crushing
strength of artificial stone is 7,500 psi.
b. Mixing. Artificial stone is mixed much like plaster of Paris. The average
mixing ratio is 30 cc (cubic centimeters) of water to 100 grams of stone powder. This
ratio may vary with different manufacturers. The required amount of water is placed in a
rubber bowl. The stone powder is added slowly. (Incorporate all of the powder with
water before spatulating.) Spatulation should be thorough without whipping the mixture.
Whipping can trap air bubbles, thus weakening the cast. The bowl should be vibrated
during the mixing to make air bubbles rise to the surface. Spatulation should be
completed in 30 to 60 seconds; after that, the bowl should again be vibrated. The use
of mechanical spatulation helps to reduce air bubbles.
c. Setting Time. The initial setting time for artificial stone is usually 8 to 10
minutes. The final setting time is 25 to 45 minutes depending on the type of stone
mixed. The surface hardness of artificial stone can be increased by soaking the cast for
several hours in a solution of borax.
Section II. DENTAL WAXES
Many different waxes are used in dentistry. The composition, form, and color of
each wax are designed to facilitate its use and to produce the best possible results.
The discussion in this lesson is limited to aspects of clinical interest.
a. General. Inlay wax is used to prepare patterns. These patterns are
reproduced in gold or other material in the fabrication of inlays, crowns, and fixed and
removable partial dentures. Inlay wax is sometimes called casting wax.