1. DENT105, Dental Anatomy and Morphology
INTRODUCTION TO
GYPSUM PRODUCTS
DENT105, Dental Anatomy and Morphology
Stephen C. Bayne
Cariology, Rest Sci, Endodontics
School of Dentistry
University of Michigan
Ann Arbor, MI 48109
Welcome to dental anatomy. This lecture is supported as an online module to compliment the activities within the course. This PPT file will play automatically. You can download the handout, a copy of the PPT frames, and/or a self-study copy of the PPT presentation that is the script for this module. You can find all of these accessible from the general site for dental materials. [CLICK] Now let’s examine gypsum products.
In dentistry, many of the procedures require the use of a model-making material or inexpensive utility material. Gypsum products are the most commonly used for these applications.

2. Product Use in Dentistry
GYPSUM
Product Use in Dentistry
GYPSUM = calcium sulfate =
naturally occurs as dihydrate
Heat removes water and converts dihydrate to hemihydrate.
[Gypsum Powder] + [H2O]   [Gypsum] + [Heat]
Calcium Sulfate Hemihydrate
Calcium Sulfate Dihydrate
Gypsum is a naturally occurring ceramic (mineral) that is often directly visible in the landscape of the far western US [CLICK] where either earthquakes or erosion have exposed older layers of the earth. [CLICK] It appears as white strata mixed with others that form part of the color landscape surrounding the Grand Canyon region. Gypsum is chemically “calcium sulfate” that generally occurs in the dihydrate form. [CLICK] For use in dentistry, part of the water is removed by heated the material. [CLICK] Then it is mixed back with water to reform the dihydrate. This is a continually reversible reaction. [CLICK] For dental applications, the hemihydrate is purchased, mixed with water, used, and generally discarded after that. Gypsum is useful in the production of duplicates of the oral tissues as models, casts, and dies during indirect procedures. [CLICK] Procedures in dentistry can be classified as direct (occurring chairside) or indirect (requiring laboratory equipment or special conditions) as shown in the lists in the slide above.
For example, to make a crown, an impression is taken of the oral tissues. It is poured with a mixture or gypsum and water that hardens to produce a positive replica. The surface of the tooth is waxed to the shape desired. The wax is embedded into a mold making material and then removed to leave a space. A gold casting alloy is poured into the space. The room temperature crown is removed from the mold, polished, and then cemented into place.
DIRECT versus INDIRECT PROCEDURES
Dental Amalgam Inlays, Onlays, Crowns, and Bridges
Dental Composites Partial and Full Dentures
Temporary Appliances

3. Calcium Sulfate Hemihydrate Calcium Sulfate Dihydrate
GYPSUM
Setting Reaction
[Gypsum Powder] + [H2O]   [Gypsum] + [Heat]
Accelerators
Retarders
[CaSO4-(1/2)H2O] + [(3/2)H2O]   [CaSO4-(2)H2O] + [Heat]
Calcium Sulfate Hemihydrate
Calcium Sulfate Dihydrate
The chemical reaction for gypsum setting is relatively simple, but the actual microstructural events are more complicated. Gypsum powder (calcium sulfate hemihydrate, CaSO4-1/2H2O) is mixed with sufficient water to promote reaction and additional water is included to insure that the mixture is sufficiently fluid. [CLICK] The crystals of calcium sulfate dissolve in the water, chemical react with the water, and precipitate our dihydrate crystals. [CLICK] To control the reaction, accelerators and retarders are normally added. [CLICK] The new crystals become interlocked and form a solid. The chemical reaction generates heat. Unreacted water evaporates from the solid. The final solid is porous between crystals.

4. Whip Mix Corporation PO Box 17183 Louisville, KY USA 40217-0183
GYPSUM PRODUCTS:
Plaster
Stone, Cast Stone
Improved Stone, Die Stone
Specialty Stones
Investment Materials
Working
Cast with
Removable
Dies
Edentulous
Cast
Orthodontic
Model
Working
Gypsum is produced for a plethora of large commercial operations (e.g., American Gypsum for wall board) but there is only one major supplier of gypsum materials for dental products (Whip Mix Corporation, Louisville, KY). If you click on the picture, you will be linked to their website.
[CLICK] While there are several categories of products, the primary applications involve materials suitable for making plaster models, stone (or cast stone) working casts, and improved stone (or die stone) for making removable dies (of prepared teeth). [CLICK] Plaster models do not require great strength. They are created only for reference of intraoral dimensions. [CLICK] Casts are used for reference for laboratory procedures. [CLICK] The prepared teeth which are to be restored are constructed specifically of die stone to resist wear and abrasion forces during fabrication and adjustment of final restorations.
Removable
Die with
Waxed Inlay

5. GYPSUM Setting Mechanism Dissolution of hemihydrate
Precipitation of dihydrate
Dissolution of hemihydrate
Slurry water
The actual gypsum powder includes crushed crystals. The size distribution and shape of the powder particles determines the relative amount of water that is required to make a suitable mixture. Only a small amount of water is required to complete the reaction. [CLICK] The new crystals which are formed tend to be elongated [CLICK] and invariably push on each other creating minor expansion. [CLICK] The more water that remains in the matrix zone between the growing crystals, the easier it is for impingements to create expansion.
[CLICK] Once models or casts are formed, they are trimmed to remove excesses and create a regular geometry. This is accomplished on a model trimmer (see the picture above). Water is used as a coolant and to collect gypsum powder debris. This is washed into a reservoir in a connected sink. This debris is called “slurry water” and may be used as a small addition to a new mix to accelerate the setting by providing seed crystals of calcium sulfate dihydrate.
Crystal expansion and interlocking

6. GYPSUM Setting Stages Mixing Time 00:00:00 00:01:00 Working Time
00:07:00
Setting
Time
00:11:00
Mixing
Interval
Working
Interval
Setting
Interval
TIME
Final Set =
Setting Time
Initial Set =
Working Time
The stages of gypsum setting are monitored in terms of the clock time from the onset of mixing ([CLICK] mixing time, [CLICK] working time, [CLICK] initial setting time, final setting time) or the length of the intervals ([CLICK] setting interval, [CLICK] working interval, [CLICK] setting interval) as shown above. [CLICK] Actual working intervals are a function of the efficiency of mixing (mixing rate, mixing procedure and pressure, temperature, …). As the reaction is accelerated or retarded the working interval decreases or increases, respectively. As the reaction proceeds to the point of developing a three-dimensional matrix, the onset of setting occurs. [CLICK] This is detected by the generation of heat or with the use of a simple penetrometer called a small Gilmore needle. [CLICK] The penetrometer has a large tip with a minor weight attached (low stress = low weight over large surface area). The tip is placed onto the setting mass. As soon as the testing of the surface no longer allows penetration, the beginning of setting interval is recorded. As the material continues to react, its yield strength (resistance to indentation) becomes greater. The completion of setting is defined as resistance to large Gilmore needle penetration (high strength = high weight divided by low cross-sectional area). [CLICK] The best clinical clue to the onset of setting is the loss of gloss. This occurs because water is being consumed by the reaction and is no longer available to fill all the spaces between the crystals and create a glossy appearance.
LOSS OF GLOSS
Large
Gilmore
Needle
Small

7. MANUFACTURE of Dental Gypsum PLASTER STONE DIE STONE
Chemical Name: -calcium sulfate -calcium sulfate -calcium sulfate
Formula: CaSO4-(1/2)H2O CaSO4-(1/2)H2O CaSO4-(1/2)H2O
Powder Shape: Irregular Uniform Uniform
Density: Porous Moderately Dense Dense
Production Steps: Heat to 115C Heat to 125C Heat to 100C
in air with steam pressure in CaCl2 sol’n
Dental Products: Plaster, Stone, Improved Stone,
Impression Plaster Investment Die Stone
Common Names: [Plaster of Paris] [Hydrocal] [Densite]
Plaster, stone, and die stone are all chemically the same, calcium sulfate hemihydrate crystals. However, the crystals are slightly different in geometry and packing efficiency. [CLICK] Plaster crystals are produced by heating gypsum to 115°C in air. Crystals are irregular, porous, and do not pack well together. Therefore, making a useful mixture requires more water that for stone. [CLICK] Stone crystals are created by heating gypsum to 125°C under pressure. Their more regular shape allows better packing, requiring less water for mixing. [CLICK] These require the least water for mixing. The strength of the set material is strongly related to the residual porosity of the solid. Plaster has more porosity because of the high level of water that was originally required for mixing.

8. Shown above is an example of a plaster product available from Whip Mix called “laboratory plaster.” The setting properties [CLICK] [CLICK] and general properties [CLICK] are listed in the table to the left. Along the left-hand boundary of the web page is a long list of other products from the same site. The orthodontic model shown to the right-hand side is typically what this material is used to create.
WHIPMIX

9. PROPERTIES of Gypsum Products PLASTER STONE DIE STONE
Typical Water Content:
> Reaction Water 18 cc 18 cc 18 cc
> Extra Water 32 cc 12 cc 6 cc
TOTAL Water = 50 cc 30 cc 24 cc
Powder = 100 gm 100 gm 100 gm
W/P Ratio =
Setting Time: 11 min 7 min 6 min
Setting Expansion: 0.20% 0.10% 0.05%
Min. Crushing Strength: 1,600 psi 3,000 psi 5,500 psi
Porosity: 35% 20% 10%
Properties and water requirements for plaster, stone, and die stone are shown above. These are very important and should be memorized! [CLICK] For chemical reaction of 100 grams of calcium sulfate hemihydrate to dihydrate, 18 cc of water is required. Additional water is required for mixing. Average powder-to-liquid ratios (P/L) are shown above. [CLICK] For plaster 50 cc of total water is normally used. For stone, 30 cc of water is used. For die stone, 24 cc of water is used.
[CLICK] As the amount of water is decreased, the net expansion of the setting material is smaller. Values of 0.2 to 0.05% are very small and provide great accuracy for the cast products. As the amount of water decreased, [CLICK] the amount of residual porosity is smaller, and the [CLICK] final crushing strength is greater. [CLICK] The crushing strength or compressive strength is the resistance to fracture from pushing on the material as schematically shown in the figure above. This will be elaborated later in your dental materials science course when we examine mechanical properties.

10. Shown above is a list of possible products for use as die stones
Shown above is a list of possible products for use as die stones. Hard rock is identified as a typical product. [CLICK] Microstone is the product that you are using for the exercises in Dental Anatomy. [CLICK] Click on the tab to go to the website. Otherwise, the module will proceed. [Wait 5 seconds.]

11. MANIPULATION of Gypsum Products Proportion P and L
Transfer to impression
Microstone
Pre-packaged P
Bulk P
Gypsum products can be supplied to dental offices or dental laboratories in large quantities (in bags, cartons, or barrels), intermediate sized containers (tubs or plastic jars), or pre-proportioned bags. [CLICK] Large quantities require manual proportioning (by weight of powder and measurement of water). Large containers are very susceptible to pre-reaction with water in the atmosphere during opening and closing of the containers.
Mixing normally occurs in a flexible rubber bowl. [CLICK] Water is placed into the bowl first and then powder is sifted through the water to facilitate wetting of the individual powder particles. A stiff-bladed spatula is used to strop (push hard against the side-wall) against the mixing bowl while rotating the bowl in one’s hand. After about 1 minute, a smooth glossy mixture is generated that is ready to be placed into an impression or other mold. The mixture includes incorporated air bubbles that need to be removed at some point. [CLICK] Material is scopped with the stiff spatula and placed into a molar portion of the impression while vibrating the impression so that (1) bubbles are removed, and (2) the mixture flows into the next portion of the impression. This will be demonstrated by the instructor in a few minutes.

12. Procedures for Gypsum Products
INFECTION CONTROL
Procedures for Gypsum Products
Impression Tray
Impression
Material
Infection Control Approaches (Strategies):
Additives in “impression material”
Immersion of “impression material” surfaces
Additives in “gypsum products”
Dental Arch
Gypsum products are generally cast into an impression [CLICK] that could contain bacterial or viral contaminants. [CLICK] Impressions are disinfected, but the process is not sufficiently consistent to guarantee that there is no risk to laboratory technicians. Some impression materials and gypsum products contain disinfectants. Lab technicians routinely wear gloves to protect themselves from persistent bacteria such as Hepatitis C. Cases are on record of hepatitic C transfers on casts which have not involved disinfection procedures that are as much as 6 months old.

13. HAVE FUN !
Gypsum products are inexpensive mold-making materials that have been used in dentistry since the 1850s because of their low cost and ease of mixing. [CLICK] Have fun working them.
THANK YOU