| Consistency | This refers to the ratio of water to plaster and is the most basic and comprehensive means of classifying plaster. The water:plaster ratio determines strength, absorption, hardness, porosity, setting expansion, and other physical characteristics of set plaster. |
| Absorption | A continuous rate of absorption – the ability of gypsum to take on water – is very important in the slip-casting process. An abnormally fast absorption rate (otherwise known as blocking) causes weak and sticky casts, surface imperfections, and casts that pull away from molds. The absorption rate is affected by the pouring consistency, mixing procedure and drying of the molds. |
| pH | Gypsum is neutral: neither alkaline nor acid in reaction. |
| Density (loose) | Gypsum rock in solid form has a density of approximately 140-145 pounds per cubic foot. Crushed gypsum rock has a density of approximately 80-145 pounds per cubic foot. In calcined gypsum (Plaster of Paris) the density is 55-70 pounds per cubic foot. |
| Melting point | Gypsum does not have a true melting point, as it decomposes under heat before it can melt. |
| Index of refraction | For gypsum, this is defined as 1.520 to 1.529. |
| Color | Gypsum is white or colorless when pure. Impure varieties range from gray to black, light pink to red and brown, pale yellow, and pale blue. |
| Streak | The color of finely powdered gypsum, which is white except in very impure varieties, when it may be tinted gray or brown. |
| Transparency | Pure selenite is transparent, alabaster is translucent, and other forms are opaque. |
| Solubility | Gypsum is soluble in nitric and hydrochloric acids. It is slightly soluble in water; 100 parts of water at 20°C will dissolve 0.222 parts of gypsum. Gypsum is almost insoluble in sulfuric acid. |
