Firing with Cornstarch on the Clay Surface

[Barbara Hanselman]

I like to use cornstarch with my classes to keep my flexible rubber stamps from sticking to the clay surface but a kiln tech at one of my art center studios told me that cornstarch residue is hard on the kiln elements when bisque firing. Does anyone have info on the effects of using cornstarch on greenware? Does the cornstarch adversely affect the kiln elements? How can I find out if it is bad for the elements?

Thanks, Barbara

[Erik Evans]

I used to be a kiln tech.  In my experience, techs are notorious for being overly cautious.  That may be the case here.  That's not to say cornstarch is completely safe.  Frankly, I'm not sure.  I'd call the folks at Euclid's and speak with them.  They're really knowledgeable and super helpful – much smarter than me.  If it is corrosive though, it's probably negligible, particularly if you're only firing dusted pots occasionally.  A significant portion of your load, covered in starch every time you fired, might be a different issue, but I doubt that's what's happening.

The only issue I had with cornstarch was build-up in nooks and crannies, obscuring the details.  Dusting with a cheesecloth punch bag helped. 

[Barbara Hanselman]

My cornstarch is housed in cornstarch socks, you know those single lost socks from the dryer who never seem to have a mate.  We apply it lightly before impressing the rubber stamps and once dry, I usually use a soft dry brush to brush off the excess.This is one example of the remaining cornstarch on a molded bowl.

Erik Evans said:

I used to be a kiln tech.  In my experience, techs are notorious for being overly cautious.  That may be the case here.  That's not to say cornstarch is completely safe.  Frankly, I'm not sure.  I'd call the folks at Euclid's and speak with them.  They're really knowledgeable and super helpful – much smarter than me.  If it is corrosive though, it's probably negligible, particularly if you're only firing dusted pots occasionally.  A significant portion of your load, covered in starch every time you fired, might be a different issue, but I doubt that's what's happening.

The only issue I had with cornstarch was build-up in nooks and crannies, obscuring the details.  Dusting with a cheesecloth punch bag helped. 

[Norm Stuart]

Like paper in paper clay, corn starch is made up of carbon, hydrogen and oxygen - all of which off-gas during the bisque or other firing. If your "kiln tech" is concerned about a little corn starch, he should be absolutely terrified by paper clay. But the kiln is designed to handle both by allowing enough air flow, so his concerns are groundless.

Firing a kiln with greatly reduced oxygen supply (a reduction firing) does corrode electrical heating elements, by removing the protective layer of oxidation from the aluminum in the Kanthal wire - which again, is why electric kilns are designed to allow enough air flow to prevent a reduction firing.

°°°°°°°° You can purposely create a reduction firing under a large upside down bowl with a charcoal briquette inside. The corderite shelves are totally resistant to the effects of the reduction firing, but the ceramic bowl (the saggar) won't last too many firings.This is why real saggars are made of fire clay which resists the reduction better.

After a successful reduction firing the charcoal briquette will be partially consumed eliminating the oxygen under the bowl and your ware inside the bowl will have experienced a reduction firing. The inside of the bowl will be eroded and possibly cracked. As mentioned, the bowl won't last too many firings.

If the bowl were to break-open  during the firing, the briquette will be almost instantly consumed by the excess oxygen in the kiln and the ware under the bowl will experience an oxidation firing.So if there is nothing left of the briquette but ash, too much oxygen was able to get under the bowl and you experienced a normal oxidation firing.

[Barbara Hanselman]

Thanks, Norm, for the explanation. We also fire paper clay so the comparison should hit home for him.

 

[Erik Evans]

Awesome!  There's your answer, Barabara.  I'm going to build a website called, "AskNormStuart.com" or maybe "NormKnows.com" because this man is just a wealth of knowledge.  We're lucky he shares with us.  As always, thanks, Norm!

[Barbara Hanselman]

Yes, he seems to be! We are lucky to have him and his input on Electric Cone 6!