10 Gypsum rules

1.Preparation

For the best results it is essential that the equipment is clean and free from residue before mixing the gypsum. Any residue would have a negative effect on the setting time and expansion of the mixture. The gypsum should be mixed under vacuum, if possible, and to the exact water/powder ratio.

The gypsum should be sprinkled into the water. Follow the manufacturer’s instructions for mixing time and speed. Measuring the water/powder ratio by rule of thumb will automatically lead to considerable variations and poor results.

2. Mixing water

Tap water, which has been allowed to sit at room temperature, can normally be used for mixing. It is safe to use demineralised or distilled water. Quality may be affected if other liquids are used, such as water from the trimmer or gypsum hardener liquids.

3. Sprinkling

The gypsum must be quickly and evenly sprinkled into the water within 10 seconds. In accordance with EN ISO 6873, timing starts whenever the gypsum and water come into contact. After the gypsum has been soaked for 20 seconds, it can be mixed with a spatula. Mixing in a vacuum mixer improves the quality of the mixture and saves time. Manual mixing requires approx. 60 seconds and mechanical mixing 30 seconds.

The vacuum should not be set too high when mixing. Impression plaster (type 1) is always mixed manually for 30 seconds. Adding water or gypsum when the consistency is too thick or too thin affects the setting process and damages the crystalline structure of the gypsum.

4. Pouring the impression

Only the amount required for two to three impressions should be mixed at a time. The impression should be poured immediately after mixing. Pouring the impression should be completed within the working time. The gypsum begins to crystallize at the end of the working stage. No further work can be carried out because fine details can no longer be accurately reproduced after setting begins.

The strength of the gypsum is also greatly reduced. This even happens when using a vibrator, though its use always greatly reduces blows and improves compressive strength and flow. Vibration should be completed before the setting stage.

5. Cutting and chaping gypsum

Gypsum have an exact setting time. If hard stone has a setting time (final set) of e.g. 10 - 12 minutes, it can be worked on for approx. 5 minutes (approx. half the setting time).

When the surface of the stone loses its shine, it can still be shaped for approx. 1 minute. After this the setting time begins. At this point no further work can be done, as this would affect crystallization.

6. Removing the model

The set model must not be removed from the impression until 30 minutes after pouring. Following cleaning, disinfecting and neutralising, alginate and hydrocolloid impressions should be poured immediately with gypsum because they are not volumetrically stable.

The impression should be removed after 30 minutes because these impression materials react aggressively with gypsum. It is advisable to leave other impression materials for up to an hour before removing them.

7. Setting expansion

All gypsum expand at the final setting. The amount of expansion depends on the composition of the gypsum and the ambient temperature or humidity. According to EN ISO 6873, comparable measurements of expansion can only be made under the same conditions. Please note that the percentage expansion of the gypsum must be measured after two hours. Compressive strength is measured in N/mm2 after one hour. The standards given on page 6 are compulsory.

Ensure that standards and times given are accurate when making comparisons. In practice, some expansion is necessary to compensate for the contraction of other materials. Shrinkage occurs if the model is stored at room temperature and low humidity over a longer period. If the model is soaked, as it is sometimes necessary, expansion of the gypsum increases slightly. Our gypsum are well below the expansion values allowed by the DIN standard.

8. Preparing the impression

Problems continually arise in the laboratory between different impression materials and gypsum. Since some impression materials react with gypsum, laboratory assistants must complete the following preparations to obtain an accurate gypsum model with a smooth surface.

Material	Alginate	Polyether	Hydrocolloids	A-Silicones
Charac-teristics	moist, soft, delicate, fragile	Affinity to water, hydrophilic (water drops disperse)	like Alginate, distinguishable by water tube on	water-repellent hydrophobic (water drops form a ball)
Properties	Shrinkage due to loss of moisture. Cannot be store indefinitely. Can be kept moist for max 1 hour	Not suitable with heavy bleeding or salivation because of hydrophilic properties. Do not store moist or together with alginate	Pour immediatly to avoid considerable volumetric changes	No volumetric changes dimensionally stable, robust, N.B. C-Silicones - can only be stored for 6 hours
Preparation	Complitely remove saliva and blood residue. Neutralise by putting into trimmer water or gypsum powder (for 3 min only or risk of water uptake)	Remove saliva or blood residue under running water. Wait at least 3 hours before pouring. Do not dry completely to avoid the gypsum soaking into the surface.	Complitely remove blood and saliva residue	Remove blood and saliva residue under running water. Wait at least 3 hours before pouring.

9. Sawing and preparation

It is advisable to saw, prepare and trim dental arches within two hours of removing the impression. If models have to be sawn or prepared much later than that, they can be soaked briefly to prevent gypsum chipping off.

Tip: Placing old gypsum models in the water saturates it with potassium sulphate to prevent washing out.

10. Boiling out - steam-cleaning - cleaning

These standard laboratory procedures, which are often unavoidable, must be carried out very carefully. Gypsum models should never be exposed to sudden changes in temperature.

We strongly recommend immersing the model in warm water at approx. 50°C for a few minutes to avoid chipping or even completely destroying the model. Steam-cleaning the model can also lead to surface abrasion. In many cases it is better to clean the model using a soft brush and a soap solution.

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