Description
Crocus Martis is the trade name for anhydrous ferrous sulphate (FeSo4) and is also known as calcined copperas. This is a material that has traditionally been used as a substitute for red iron oxide, and is often not as soluble as iron oxide -- as a result, this can result in specking in glazes, which may be a desirable effect for some. Please note that this material releases sulphur during firing, and may not decompose completely until 1200C or higher.
Historically this was a natural ocurring mineral with varying chemical compositions. Now it is a commercially reproduced to approximate an interpretation of the natural mineral. Because of this, we recommend testing each glaze recipe mixed with this commercially developed version.
