Anyone tried firing standard #563 clay to cone 5?

[Camiev]

#563 is a cone six clay and I was wondering if anyone tried firing it to cone 5, or in general what happens when you fire clay a cone above or below its type.

[Callie Beller Diesel]

Hi and welcome to the forum!

So much of what can happen depends on the formulation of the clay body in question. I haven’t worked with Standard clays specifically, but I can give you some generalizations while we wait for someone who has to chime in.

As a rule, if you’re firing a cone 6 clay only to cone 5, it won’t be as vitreous or mature as expected, or will be on the lower end of the range the manufacturer gives. If you’re making functional ware, you do want to be mindful of your absorbency rates so you don’t wind up with pots that weep or get mildew stains on the unglazed portions. Even if a manufacturer gives you an expected fired absorbency, it’s a good idea to make a test bar and run your own test to verify you’re reaching <0.5% with your firing cycle. Here’s a link to Digitalfire with test instructions, and some expanded reading on why this is a good idea.

If you overfire any clay body, at some point it will bloat, and if you go far enough, it’ll melt entirely. What point that happens at will depend on the clay body. Some have more wiggle room than others. Most pottery kilns won’t generate enough heat to fully melt a cone 10 stoneware, but the results of putting an earthenware into a cone 10 firing has made many a kiln tech get very, very inventive with their adjectives.