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Dental Materials
Gypsum and Wax
| Question | Answer |
|---|---|
| What is the chemical makeup of the mineral gypsum and how is it mined? | Chemically, the mineral gypsum is a dihydrate of calcium sulfate and is mined as a solid mass. |
| Is the amount of water required to mix the calcium sulfate hemihydrate greater or less than the amount required for the chemical reaction? | greater |
| How is the raw material of gypsum formed into a powder? | the manufacturer heats this dihydrate, which causes it to lose water |
| When the hemihydrate powder is again mixed with water, what is then produced? | a product with a viscosity capable of flowing is produced. |
| Which type of dental gypsum products will require the least water for mixing and will have the highest final density? | Dental stone, high strength (Type IV) |
| What is a model plaster used for? | diagnostic models |
| What is another name for β-Hemihydrate? | model plaster |
| What is dental investment? | a material able to withstand the high heat and stress produced when molten metal is forced into molds to form indirect restorations |
| What are the characteristics of die stone gypsum powder? | smaller, more irregular, and denser particles |
| What is dental stone type III used for? | denture models, orthodontic models or casts |
| What is needed to convert the hemihydrate particles back to dihydrate particles when mixing gypsum products? | Increased porosity of the gypsum particles makes it necessary to use more water |
| What happens to the cast if more water is used? | the more water that is used, the weaker the cast will be |
| How does it affect the product if less water is used? | it will have a higher density of crystals and is therefore a denser and stronger product. |
| What type of gypsum product has the least amount of setting expansion? | High-strength stones |
| What is the water-to-powder ratio of model plaster? | 0.45 |
| What is the water-to-powder ratio of dental stone type III? | 0.30 |
| What is the water-to-powder ratio of dental stone type IV? | 0.23 |
| What is the water-to-powder ratio of dental stone type V? | 0.20 |
| What is dental stone type IV , otherwise known as improved or high strength stone, used for? | used to fabricate crowns and bridges |
| How are working models for cast restorations often made? | a die of type IV is poured with a base and adjacent teeth of type III stone |
| If a diagnostic model is being created, what would be the gypsum of choice? | plaster |
| What is the correct way for mixing gypsum products? | water is placed into the mixing bowl first and the powder is slowly sifted into the water. |
| What will lengthen the setting time during mixing? | decrease the spatulation time and rate |
| What will increase the rate of setting expansion during mixing? | increase the spatulation time and rate |
| What does loss of gloss mean for newly mixed gypsum products? | indicates it has reached its initial set and the working time has now begun |
| How long does it take for the initial set to occur after beginning to mix the gypsum? | 8-16 minutes |
| How much mixing time does the dental assistant or dental hygienist have when working with gypsum products? | 1 minute |
| What is happening when a gypsum model is setting and is giving off heat? | exothermic reaction |
| Explain the double pour method. | It involves two separate mixes. The anatomic portion of one or both arches is poured and left in the upright position and a second mix is produced for the art portion. |
| How should a diagnostic cast should be poured? | The art portion is free of all air voids greater than 2 mm and the anatomic portion is free of all air voids. |
| When evaluating the diagnostic cast, what does a well-done cast look like? | The occlusal plane at the premolar is parallel to the bottom of the base, the art and anatomical portions form a continuous surface and sufficient material extends past the mucobuccal fold and posterior teeth |
| What happens to the viscosity of the wax as the temperature decreases? | it increases |
| Which types of wax are used for indirect restorations? | inlay wax and casting wax |
| What are the properties of corrective impression wax? | Used for edentulous impressions in conjunction with other impression materials. This wax flows at mouth temperature and is used to correct undercut areas or to fill in small voids. |
| What are some examples of processing wax? | sticky wax, utility wax, and boxing wax |
| How is inlay wax used? | it is used directly in the mouth and placed onto the prepared tooth in the direct waxing technique |
| What are the characteristics of inlay wax? | They are supplied in sticks, pellets, and tins, generally in dark colors of blue or green. |
| How is type II inlay wax used? | melted onto a die outside the mouth in an indirect technique which most dentists prefer |
| Which kind of wax is used to construct the metal framework of partial and complete dentures? | casting wax |
| What type of wax is used mainly to make occlusion rims and to hold artificial teeth to baseplates during the fabrication of dentures? | baseplate wax |
| What are the characteristics of boxing wax? | Boxing wax is slightly tacky at room temperature which allows it to adhere to itself. A strip of boxing wax is 1.5 inches wide. This wax is easily manipulated at room temperature. |
| What is utility wax used for? | to adapt the periphery of an impression tray, to cover sharp brackets and wires for orthodontic patients and to provide a better fit into the vestibule of the impression material. |
| Which type of wax is used to adhere components of metal, gypsum, or resin together temporarily during fabrication and repair? | sticky wax |
| In the lost wax technique for casting gold for dental restorations, what is attached to the pattern to form the channel into which the molten metal will be forced? | sprue |
| What are some uses for gypsum products? | die, casts, study models |
| What is the term for the heating of gypsum products to remove water? | Calcination |
| What are the differences in the physical properties between plaster, stone and improved stone? | They differ in their powder particles as a result of differing calcinations methods, making them suitable for different uses. |
| What are the properties of gypsum products? | easy to use, accurate, ability to produce fine detail |
| What are the properties of plaster? | Plaster is usually white, the least expensive and is the weakest. |
| What are the properties of stone? | Stone is usually yellow in color but it can also come in other colors. It is stronger than plaster. |
| What are the properties of improved stone? | It is the most expensive of the dental gypsum products and it has the greatest strength so it is used for making casts or dies for crowns and bridges. |
| Which materials could be used to allow the gypsum material to set up slower? | Borax and water |
| Which materials could be used to allow the gypsum product to set up faster? | Potassium sulfate and water |
| What would be the cause of “bumps” on a study model? | voids in the impression |
| What is the procedure for pouring up an impression with gypsum material? | Dispense the water by volume in a graduated cylinder, weigh the powder in grams with a scale, fill the impression tray by flowing the material slowly with a dental vibrator |
| What does increasing the mixing time of gypsum products do? | decrease the setting time |
| What is a common accelerator to decrease the setting time of gypsum products? | potassium sulfate |
| What products could be used to allow the gypsum material to set up slower? | Borax and water |
| What is the working time in a setting reaction when mixing gypsum materials? | The length of time from the start of a mix until the mass reaches a semi-hard state, the available time for manipulating the material, indicates a partial progress of the setting reaction |
| Which is the most expensive of the gypsum products? | High strength stone |
| Which is the least expensive of the gypsum products? | plaster |
| What is the difference between stone and plaster? | Stone is less porous, stronger, and more expensive than plaster. Stone can be yellow in color. |
| What would be an example of when you would NOT want to use plaster? | when strength is a requirement such as for working models. |
| What are the properties of gypsum products? | Easy to use, accurate, ability to produce fine detail |
| Why do plaster, stone, and improved stone differ in the physical properties of their powder particles? | it is a result of differing calcinations methods which make them suitable for different uses |
| What are the uses for gypsum products? | Die for single teeth, cast, study models |
| What is occurring when the newly mixed gypsum material loses its glossy appearance? | The working time has begun |
Created by:
BrendaAlberts