You know how you're looking for one thing and get side tracked with something else? I saw this Clay Club post for Magruder's Red glaze and read that it contained synthetic iron oxide. Synthetic? How could iron oxide be artificial? Anyway, on my way to finding out, I came across this article about alternative glaze materials by John Britt, which is a nice coincident since he also wrote the Clay Club post:
One of the materials that grabbed my attention, obviously, was toothpaste. Imagine opening a glaze bucket to that minty fresh smell. Now when people ask me about glaze materials I can tell them something they can relate to. Usually they look like they regret asking right around 9 seconds into my explanation.
Back to synthetic red iron oxide- Here's Webster's online definition: a pigment that is produced from an iron salt (as copperas) by precipitation or calcination under controlled conditions and is often purer than natural iron oxides.
And, a little more googling told me that natural red iron oxide is rated 85% pure, Spanish Red is 83-85% pure and synthetic is 96-99% pure. Crocus Martis is a brand name for synthetic iron oxide. I've come across this before in glaze recipes but did not know it was "synthetic". I'm glad for this little education because a few years ago I wanted Crocus Martis and when my local supplier didn't have it they suggested substituting Spanish Red. I used it and the glaze was supposed to be in the red range, but turned out more brown than red despite my attempts with cool-down cycles and holds. John Britt also gives a good tip for a source of synthetic iron oxide in the article on alternative glaze materials- cement pigment.