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CAS Numbers

CAS (Chemical Abstracts Service) is a registry of chemical identification numbers maintained by the American Chemical Society. Although ceramic materials are generally minerals or processed minerals rather than chemicals, most do have numbers in the system. However, some numbers do not have a high level of specificity (e.g. there is one number frits but there are many kinds of frits). Likewise for clay (although there is a specific number for kaolin). Feldspar has one number but there is a special one for spodumene (lithium feldspar).

Assigning CAS numbers to materials can be tricky and a matter of judgment. For example, brick grog has no CAS number, but since it is fired clay, and thus contains quartz particles, it could be classified as a clay. That being said, some companies use the CAS numbers for alumina and silica (this is not accurate, while it contains Al₂O₃ in the crystal structure, it does not contain any pure crystalline or amorphous alumina). Many natural materials are mixtures of minerals, for example, Gerstley Borate is ulexite and colemanite. Since it has no dedicated CAS number, those two can be used. Bentonite is a general term for high surface area clay and it has a CAS number. But it can contain various high surface area minerals (e.g. montmorillonite, smectite, hectorite) and each of these has its own CAS number. Companies use their judgment about which to use. There is a general CAS number for clay and also for kaolin, almost all clays contain both (although it can be a judgment call in identifying which a particular is). Recipes (e.g. clay bodies, glazes) normally contain silica, clay, feldspar, frits, etc, so it seems advisable to include the CAS numbers for each. Frits have one general number but there are many for stains, depending on their type, it can be quite difficult to find the right one. Metal oxide colorants tend to have different numbers for the oxide, carbonate and hydroxide versions. Borax has many forms and can be tricky to find the right CAS number. Liquid materials can be tricky to number since they can have many ingredients. Strangely you might actually find water, with its CAS number, listed on some data sheets!

It is easy to google a number, e.g. "CAS number feldspar". While this often reveals one right in the search results, it may not be right. CAS numbers are often deprecated, upon finding the up-to-date one it can be surprising how many it replaces (e.g. SiC has more than 30 deleted or replaced numbers). And it is also very common to find safety data sheets having incorrect CAS numbers.

CAS numbers make it easier and faster to search online databases since the name of a given material can have many forms. Since regulatory bodies in many countries require that companies and individuals keep good records of the nature and hazards of the materials they have on hand, reference to these numbers is valuable. That being said, caution is needed regarding the CAS number printed on some original container bags, it may not be the most applicable one (for reasons we can only speculate). Numbers shown in safety data sheets also conflict, different suppliers often quote different numbers on hard-to-classify materials. It is not unusual to find deprecated ones or multiple numbers quoted for a material (these refer to constituent minerals within).

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