Chalk is a type of very fine limestone, which is formed from microscopic shell parts from fossil algae and their skeletons. Chalk deposits can be seen on, for example, Møn's or Stevn's cliff today. The chalk is muddied (cleaned) with water, after which it can be processed further. Chalk is used, for example, in cement production and as a general filler in many different types of paint. See below.
In white or slightly tinted glue colours, chalk is the main ingredient, as chalk in water added to e.g. skin glue, bone glue, carrageenan or painter's glue (methylcellulose) gives a good covering colour. Supplementation with titanium white will increase the coverage.
Linseed oil putty
Chalk can be kneaded with linseed oil to make a window putty.
Linseed oil paint
In linseed oilpaint, chalk can be used for thickening, so that you can work with a slightly fuller layer without the paint wrinkling. But if you add chalk to a linseed oil paint, it reduces the quality of the paint. Especially outdoors, the paint will dull, weather too quickly and require re-treatment. It is better to add more pure pigment or reduce the amount of oil to get the right viscosity than to add too much chalk.
Can be used to remove excess oil residue on the glass when you putty windows with linoleum putty.
Shade lime in greenhouses
Chalk mixed in water can be used on the glass in the greenhouse to keep more light out and thus avoid burning the plants.
As a filer
As the main ingredient in Gesso for priming canvas before a painting begins.
Chalk must be stored dry.
Chalk is described as non-toxic. However, it is always advisable to use a dust mask when working with it.