Zahn Dental Sales - March/April 2018 - 31

Understanding Dental Gypsum
A Dental Lab Professional's Guide to All Things Gypsum
Dental gypsum is one of the most commonly used materials in the dental
laboratory. It is manufactured in many colors, strengths and with many
different degrees of expansion. Though every dental lab uses at least
one, few technicians understand the differences and the effect they can
have on the final restoration or prosthetics.

8 Rules to Follow When Selecting a Gypsum:

Dental gypsum is separated into 5 different types. Understanding how
they are to be used will help determine which type you should choose
for any given application. Every gypsum product fits into a specific type,
as defined by the ISO (the International Organization for Standardization).
That rating categorizes the material in terms of its properties, i.e.
compressive strength and expansion.

4. Not all doctors perceive tight the same way. Find out where it's tight
before changing the process.

Type 1 & 2: These low strength gypsum materials are typically referred
to as dental plasters. They are for mounting models to articulators or
used as diagnostic models which will not be worked on. Type 1 and Type
2 gypsums are available in "fast" and "regular" working times.
Type 3: Sometimes called "model stone" or "specialty stone", this stone
is used for: opposing models, bases for the pinning technique, crown &
bridge applications, models for denture technique. These are chosen
for how fast you need to work on them and their expansion.
Low expansion is best for crown & bridge work, while higher expansion
is best for denture work.
Type 4: Gypsum with an ISO rating of 4 can be a specialty stone, and is
most commonly used as a die stone. If the casting fits the model, but is
a little tight in the patient's mouth, then it would be wise to switch to a
larger expanding stone, i.e. in the .12 to .15 range.
Type 5: Type 5 gypsum is a high expansion die stone, beginning at .16
and increasing. It is recommended to use this kind of gypsum when the
dentist is using impression materials that have a large linear contraction
of .25 or greater, or when a "loose" or "passive" fit in the mouth is desirable.
Compressive Strength
Because of the traditional processing of prosthetics, compressive strength
has played an important role in gypsum and is thus required reporting
by the American Dental Association.
Compressive Strength is a measurement of a material's resistance to
compression, which is created by any force applied to the stone mass.
In the removable side of dental technology, compressive stress takes
place when the prosthesis wax-up is flasked, trial packed and then final
packed and pressed using a hydraulic or axial press.
When flasking in preparation for trial packing, technicians generally follow
a rule of using a flasking stone with a lower compressive strength than
the model on which the denture or partial was originally constructed.
The technician should be aware that the combining of materials also
changes all the other physical properties stipulated by the manufacturer.

1. Find the contraction of your impression materials.
2. Use a low reported Die Stone
3. If the fits are tight, use a larger expanding die stone.

5. Make sure the impression is free from all contaminants, i.e. biological
and chemical, before pouring.
6. Use a low expansion Base Stone since mid or high expansion stones
may cause die drift, showing as ill-fitting bridges or contacts.
7. Higher compressive numbers don't mean a harder surface.
8. Combining stones changes all the properties. They can expand
or set differently.
Important Steps for Gypsum Model Preparation
Store Gypsum Wisely-Seal and Store in low humidity.
Measure Twice, Mix Once-Use water/powder ratios defined by the
manufacturer. Review the procedures on a regular basis to make sure
that you have not drifted from the intended use or handling instructions.
Clean is Best-Make sure that your areas and your equipment are clean
and in proper working order.
Beware of Interactions-Don't mix gypsum stones and plasters in the
same bowls used for phosphate investments. The residual phosphate
can keep the gypsum from setting properly.
Less is More-Make sure that your impressions are clean and only slightly
damp before you pour. Excessive use of alcohol-based surfactants or
debubblizers can also create surface issues like softness or chalkiness.
Eliminate Impression Headaches-Do your best to match the linear
expansion of your stone to the linear contraction of the impression
materials that you receive. Always check the impressions for problem
areas: Tears, voids, unset material, air traps on marginal areas, or tray
showing through material. All are potential sources of case failure and
should be pointed out to the doctor and noted on the prescription, along
with the doctor's indication to continue or re-take the impression.
Remember that what you do with your gypsum work is the foundation
of all that happens on it. Beginning with a solid and attractive foundation
helps to ensure that your restorative creativity will be appreciated by the
dentist, the assistant, and the patient.
Information provided by:

To Order Call: 1.800.496.9500

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