1. Always use a clean mixing bowl and spatula.
The best time to clean a bowl and spatula is immediately
after pouring the impression while the material is still
soft and easy to remove.
2. Measure the volume of water and weigh the
powder before you mix any gypsum material. An
accurate water-to-powder ratio is a must to preserve the
properties of any gypsum product.
3. Always add the powder to the water, never the
water to the powder.
4. Spatulate thoroughly by hand, incorporating al
the powder evenly throughout the mix until creamy
Avoid whipping the mix; doing so will cause the final
product to have excessive air bubbles.
5. Vacuum mix using a power mixer-investor
whenever possible. Vacuum mixing helps to eliminate
incorporation of air into the mix. If you can't vacuum
mix, tap the mixing bowl against the bench top or hold it
on the vibrator for a few moments to express any air that
might be trapped in the mix.
6. Never add water to a mix that is too thick; this
interferes with the setting properties. It would be better
to discard the mix and start over again.
The primary objective when you pour a cast is to
capture all the surface detail of the impression, and as
bubble-free as possible. This is done using a vibrator
to make a thick gypsum mix, which flows into all the
crevices of the impression. There are several ways to
pour impressions, such as the upright, two-step, and
boxed methods. We will only discuss the two-step
method of pouring a cast, because it is the most
successful method of pouring preliminary and final
Begin by holding the impression tray so that the
handle rests against the vibrator. Start at one end of the
arch and flow a small amount of stone into the
impression, letting it slowly advance to the other side
as shown in figure 8-30. Flow the stone slowly enough
to watch the progress of the stone as you fill each tooth
imprint. This should eliminate bubbles. If a bubble
appears and does not go away with vibration, pop it
with a small instrument. Use an acrylic mixing spatula
to place small amounts of stone into minute
preparations or teeth with wide incisal and narrow
cervixes. Touch the impression to the vibrator to flow
each addition of stone.
After covering all the critical surfaces of the
impression, you may safely add progressively larger
amounts of the mix. There is a rate of vibration that is
best for each mix's ability to flow. The vibration
intensity should be set high enough to make the
material move across the surface of the impression.
The vibrator is set too high if the impression "jumps" in
your hand, moves so fast that it skips over surface
detail, or if vibration wave patterns develop on the
surface of the mix. Continue filling the impression
stone to a level slightly above the height of the
impression walls (about 2 mm thick). Do not flow
stone over the outside of the tray because it must be
removed before the impression can be separated from
Lastly, add retention nodules to this first pour as
shown in figure 8-31(A). Stone retention nodules are
used between the first and second pours so the two
stone layers can be locked together mechanically.
Place the handle of the tray in a holding device; do not
lay the impression on the counter or the cast will be
distorted. Now let this first pour set for about 45
minutes, or at least until the stone loses its glazed
appearance before making the cast base.
It is now time for you to make the cast base. Use
the cast trimmer to grind down the long retention
nodules (if completely set), reduce the base of the
cast's thickness, and make the top of the tray parallel to
the cast's bottom. In general, the length of the retention
nodules should equal the height of the trav flanges.
Figure 8-30.Pouring the first pour of an impression.