[Clayart] Clayart Digest, Vol 44, Issue 31
claywork at flying-snail.com
Sun Jul 28 22:52:19 EDT 2019
> On Jul 28, 2019, at 3:53 PM, Roxanne Hunnicutt <roxhun at gmail.com> wrote:
> Hey, Snail,
> Is the need for higher heat, talc? Or some other dark grey material?
Generally, the grey color in unfired clay is from carbon. This needs to burn out, and its presence usually means there are substantial amounts of secondary clays in the clay body. these give the body lovely plasticity, but secondary clays are the most likely to have other impurities as well, which must also be burned out.
(Primary clays are those which have weathered and eroded down from their parent rock, but are still basically sitting in the same spot where the rock used to be. so, they tend to have mineral compositions that are very ‘pure’, a.k.a. similar to the original rock and with very few added minerals. They generally tend to be high-firing and coarse in texture, and low in plasticity. Secondary clays come from similar parent rock, but have been picked up by wind and water and transported to a more distant deposition site, picking up all sorts of hitchhikers along the way, including organic materials (leaves, algae, fish poop, you name it: it’s all carbon), as well as things like iron (the reason such clays are more likely to be red), and various other minerals. These clays are usually low-firing due to picking up extra fluxing minerals, and finer textured since bigger particles don’t get carried as far away, and highly plastic due to that small particle size. So, secondary clays are a useful component of most throwing bodies, but they may need a thorough burnout in order to prevent leftovers from outgassing during the glaze firing.
So, my rule of thumb: the grayer the clay, the more thoroughly it might need to be bisque fired. This is not an absolute, of course. Not all glazes are sensitive to outgassing, and not all claywork gets glazed, but that’s why it’s just a rule of thumb.
More information about the Clayart