Now, I have found that the amount of time the clay has been sitting in the cornstarch, prior to baking had a big effect of the clay strength when baked. (Possible leaching of plasticizers from the clay, due to absorption qualities of the cornstarch.) I found clay that went straight into the oven after immersing in the cornstarch was fine, however if it sat in the starch for any length of time it become weak.

How long did your samples sit in the starch before baking? I would be curious to know if this could have possibly been the source of the problem or if indeed it is a brand or formula issue.

I’ve been converting some of Cindy’s gorgeous Premo colour recipes to Kato, and in doing this had found that SOME of my Kato colours were shifting a bit during baking, away from violet and towards blue. So I thought I’d bake a sample immersed in cornstarch to see if that made a difference (tenting doesn’t). I mixed up one of the recipes that I’d seen colour shifting with and cut out two colour chips, each 41mm long by 28mm wide by 1.8mm thick. One went onto a 10cm white tile for baking like I normally do, and the other went into a small (150mL) clear pyrex dish with 1.5cm of cornstarch in the bottom, and was then covered with another 1cm of cornstarch. Both then went straight into a 152C/305F oven for 25 minutes (recommended baking time is 150C/300F for 10 minutes on the oldest and newest packages; the 135C/275F on some packets in the middle of that is apparently an aberration, to use my words instead of Van Aken’s), were taken out and left to cool naturally.

When I took the chip out of the cornstarch I found that it hadn’t shifted colour, but it was so weak that I could practically break it by looking at it! Well, not QUITE that bad, but almost. Very light finger pressure was enough to snap it into small pieces, and I can actually pulverize those pieces completely when I press hard. However, the chip baked on the tile was as strong as ever (taking several hundred full bends with pliers to break it, in line with what I’d noted in the temperature/strength test info I posted above).

25 minutes is two-and-a-half times the “recommended” baking time as printed on the packet, although it’s about normal for me. It was only a small dish of cornstarch so there wasn’t much extra thermal mass — certainly less than the tile the other sample was baked on — but it does have an insulating effect.

I’d said earlier in reply to Karrah’s comments that I hadn’t tried cornstarch immersion. But now that I have, Cindy’s guess that it might be a brand thing to do with Kato looks like being on the mark. I’ll do some proper testing in the next week or two and will post the results (e.g. is it possible to get decent strength when Kato is baked immersed in cornstarch; if it is, how long does it take… and does that pesky colour shift return when the strength does!) but thought I’d mention straight away that cornstarch immersion doesn’t seem to be a good idea for the Kato brand.

Good luck!