Fixative may be made of various substances, including casein suspended in alcohol, which some people suspect was used by Edgar Degas for his pastel work.
What is fixative?
Fixative is a colorless surface coating that prevents media such as chalk and pastel from dusting and smudging. It is not used by watercolor artists, but it is essential for charcoal drawing.
There are four main reasons why an artist might choose to use a fixative: 1) it keeps dirt from penetrating the surface of the artwork; 2) it adds gloss; 3) it helps bind the pigment to the surface; 4) it helps prevent fading.
However, it sometimes changes the color of pastels, particularly lighter shades containing chalk. To prevent this, spray no more than a light mist, rather than a thick coating. Fixatives may also be used between pastel layers.
Remember that acrylic fixatives are usually toxic, and wear a respirator during use. If you are looking for a less toxic fixative, opt for a resin-based formula.