2. Gypsum Products
Used extensively in dentistry to make dental models.
Obtained from natural gypsum rock (dihydrate form
of Calcium sulfate CaSO4 . 2H2O)
CaSO4 . 1/2H2O + 1 1/2 H2O
CaSO4 . 2H2O +
3900 cal/g mol
4. Use of Dental Models
Diagnosis for a ﬁxed or removable
Diagnosis of orthodontic treatment.
Visual presentation of dental
Making of custom trays.
Making of orthodontic appliances.
5. Ideal Requirements
- Sufﬁcient strength to prevent accidental
- Sufﬁcient hardness to avoid surface damage
Should produce ﬁne details and sharp margins
of the impression.
6. Should show little dimensional change on
setting and remain dimensionally stable.
Should be compatible with the impression
materials. (No interaction between the
impression surface and the model surface)
Should have good color contrast with the other
materials being used.
Should be cheap and easily used.
10. Model plaster: commonly called plaster of Paris, is
used primarily for pouring preliminary impressions
and the making of diagnostic models.
Dental stone: for use as a working model when a
more durable diagnostic cast is required. Examples
are in the making of custom trays and orthodontic
11. High‑strength stone: also known as densite or
improved dental stone. Its strength, hardness,
and dimensional accuracy make it ideal to
create the dies used in the production of
crowns, bridges, and indirect restorations.
13. Chemical Reaction
The chemical reaction takes place
the amount of water needed.
Water thick mix, difﬁcult to handle, trap air
bubbles but the set gypsum usually strong.
Water Thinner mix, porous easily handle, but
the set gypsum is inferior.
14. W/P Ratio for DS and
High Strength DS
Crystal shapes and concentration play a major
role in W/P ratio needed to obtain a workable
15. Mechanism of Setting
Different solubility between CaSO4 . 2H2O &
CaSO4 . 1/2H2O.
CaSO4 . 1/2H2O + H2O
Nucleation process ( needle-like crystals)
Bonding between contacting crystals
17. Effect of Spatulation
Deﬁnite effect on the setting time and setting
amount of spatulation
18. Effect of Temperature
Temp. of the mixing water as well as the temp.
of the environment affect the setting time in
one of two ways:
1. Change in the solubility of CaSO4 . 2H2O &
CaSO4 . 1/2H2O.
- solubility ratio
slow the reaction
20. 2. Change in the Ion mobility:
sol. & mobility of Ca++ & SO4-reaction rate
- at 100°C
rate of reaction decreased and
the the setting time is lengthened.
21. Effect of Humidity
When humidity reaches 70% and above:
the moist convert CaSO4 . 1/2H2O into CaSO4 .
2H2O which will act as extra centers of nucleation,
thus accelerating the reaction.
CaSO4 . 1/2H2O
amount of available of
retarding the reaction.
22. Effect of Colloidal
Systems & PH
Agar and alginate retard the reaction
High pH accelerate the reaction, while low pH
retard the reaction.
Setting time: the time required for the reaction to
be completed (ﬁnal setting time).
Final setting time: when the material can be
removed from the impression without distortion.
Initial setting time: the time required to reach a
certain arbitrary stage of ﬁrmness in their setting
28. Surface Hardness and
After setting occur the C.S remain constant tell
all the excess water evaporate from the surface,
after which it increase.
Hardness increase at higher rate than C.S.
Attempts have been made to increase the
hardness of gypsum products.
Impregnating with epoxy or MMA, oils,
29. Production of Details
ADA speciﬁcation requires type I & II
reproduce a groove 75 µm in width, type III, IV
& V reproduce a groove 50 µm in width.
Gypsum Vs Epoxy.
30. Setting Expansion
The % of setting expansion varies from one type
Plaster 0.2% to 0.3%.
Low to moderate-strength DS 0.15% to 0.25%.
High-strength DS 0.08% to 0.10%.
High-strength high expansion DS 0.10% to 0.20%.
31. almost 75% of the expansion observed during
the ﬁrst 24 hrs occurs at the ﬁrst hour of
The powder is added to the water and allowed
to settle for 30 sec.
Mixing can be done by hand spatulation or by
Pouring requires care to avoid trapping air.
Once poured it has to be allowed to set for 45
to 60 min.
34. Other Die Materials
Silico Phosphate Cement: Hard but shrink on setting.
Amalgam: Amalgam reproduces ﬁne details but has
Should be packed against rigid impression materials.
Needs a long time to reach maximum strength.
Needs a separating medium.
Has high thermal conductivity so it cools the wax
pattern rapidly, which may lead to pattern
35. Acrylic resin and other polymeric materials:
They have high abrasion resistance and they
are harder than gypsum but their shrinkage
during polymerization leads to inaccuracy.
Epoxy die: Epoxy die is very toxic and it
shrinks during polymerization.
36. Metal sprayed die:
Bismuth tin metal sprayed
directly on impression to form metal shell then
the gypsum is directly poured in it.
Electroplated die: Silver plated die is used
with rubber impression materials but is not
used with impression compound.
Copper plated die is used with impression