Introduction
Alginate, the irreversible hydrocolloid, is a fine powder mixed with water. It is one of the most used and least expensive impression materials. It is called irreversible because the gel cannot be turned back into a liquid.
Composition
The alginate comes from brown seaweed. In dust-free alginates, glycols coat the powder, because inhaling it harms the airways. Chromatic alginates contain a dye that changes color as pH falls from 11 to 8, showing each stage.
Setting
With 1% sodium phosphate, working time is about 2 minutes (type I, quick setting). With 3% it is about 4 minutes (type II, normal setting).
Preparation
- Shake the pack, because the ingredients separate in storage.
- Pour the measured cold water into a clean rubber bowl, then add the measured powder.
- Mix with a spatula pressed firmly against the wall of the bowl, for the time given by the maker, usually 45 to 60 seconds. This pushes out the air.
- Load a perforated tray, a tray with adhesive tape, or one coated with adhesive.
- Press it onto the moist tissues and hold it still.
- Wait 2 to 3 minutes after the gel forms, so that strength rises and permanent deformation falls.
- Remove it, rinse off saliva and blood, and pour the model as soon as possible.
Control working time with water temperature only, never by changing the ratio or mixing time. Water that is 10 °C warmer makes the reaction almost twice as fast.