Soft, beautiful colors are easy to achieve by adding pigment to gypsum plasters. The basic formula is calculated by weight. To obtain colored gypsum plaster, Ochers and pigments can be added in ratio to the dry weight of the plaster itself. The amount used will depend upon the depth of tint desired and whether the pigment is a natural one or a manufactured Oxide. To determine how much pigment to add, follow these guidelines:
It cannot be stressed enough the need to do sample testing for your colors. It is important to:
For batch to batch consistency, ratios should always be determined by weight rather than volume as pigment is like flour and the weight of any volume measurement can differ. It is also important to make sure that a sample has completely dried to see the truest color. After adequate drying time, test for any moisture in the sample by holding your hand or cheek against it. Moisture will feel cool to the touch. If a small sample board is created, move it around to different locations to see how the color looks in different light. It may look different in one room from another. Part of the beauty of pigment in plaster is how it changes with the changing light.
To mix, slake the pigment in an equal volume of water and work to create a homogenous paste. It is important that all the pigment particles be wetted to prevent dry particles that could result in "microbursting" during application. This is the appearance of specks of unslaked pigment that can streak. Sometimes minimal microbursting will occur but can be worked out with the trowel. Add the slaked paste to the wet plaster mix. The use of a mixing paddle attachment is recommended. When adding pigment color to plaster, it should be considered as a powdered aggregate. Therefore it may be necessary to adjust the water to your mix, particularly when using Ochers. Rather than adding additional water, it may be sufficient just to thoroughly saturate the substrate before application of the plaster, and if necessary, light misting of the plaster with water while it is being worked. Follow the proper application instructions for your product and type of mix.
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