• Materials for Ceramics

Crocus Martis

Description: Anhydrous Ferrous Sulphate FeSo4, Calcined Copperas

Notes

Crocus Martis is an old material name, not a dependable chemical specification. It is used as an iron colourant, often in glazes as a substitute for red iron oxide.

There are varying ideas about what this material is. Historically, it was a naturally occurring red iron oxide containing impurities (i.e. clay minerals) or even an ultra-high iron clay. The term "Crocus Martis" has also long referred to prepared iron oxides, especially red ferric oxide made by roasting or calcining ferrous sulphate (the sample analysis given here). Adequate calcination decomposes iron sulphate to Fe₂O₃ and makes it insoluble.

If the material is not highly processed or if agglomerates are not broken up, iron specking can occur in glazes. The iron content may or may not be similar to the product you have been using; don't just base substitution on the Fe₂O₃-equivalent content information from your supplier; it is better to test for differences in colour and specking in your glazes (and adjust the percentage accordingly).

If you are using a roasted product, there should be little worry about sulphur release, that happened during processing.

Related Information

Crocus martis, red iron oxide, Spanish RIO

(Left to right)

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A potter is testing these to determine the comparative coloring and surface effects on her glazes.

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