Simply Smiss asks: How do you know how long to bake the clay/know it is ready?
The worst thing that can happen to Polymer Clay is it be undercured, but you cannot really overcure it. Burning it is a completely different matter. (If you are using Sculpey III ignore what I just said.. it is frankly a PITA and can be overcured).
So what I do is use an oven thermometer to check my temperature, and I have it on the optimum temperature for the brand of clay I'm using. I preheat my oven, I find that having it sit 'on temperature' for a while seems to stop temperature spikes, which can burn your clay. Then I put my pieces in for at least double the recommend curing time unless they are really thin or really tiny. By putting them in longer you ensure they are cured thoroughly.
I have had pieces which I didnt cure for the longer time that I thought were cured and when I went to drill the holes the bead literally broke in half. Since that time I havent put any piece, other than a few minis, in the oven for anything less than 40 mins, mostly they go in for an hour.
It can be hard to determine if they are cured by sight and feel, more because Polymer Clay is still 'flexible' when its warm and thin pieces will retain flexibility even after they cool. Baking them for an extended time at the recommend temperature is all you can really do to make sure.
I preheat generally go an hour also. Unless I'm curing something that is glass, Then I put in in the oven cold and start my ''bake time" once the oven comes to temperature. This allows the glass to warm to temp also. I started doing this after I had a few salt and pepper shakers crack on me long after I made them. I don't know why but believe that perhaps the glass kept the clay from curing properly? I haven't had any crack since.
First an oven thermometer is a must! The temperature on my little oven is so far off it isn't even funny! I always preheat for a good 10-15 minutes. My oven spikes way over the target temperature, but then stays pretty stable after that. Know your oven's quirks!
I use Premo and Kato clays.
Per Premo instructions, cure at 275 degrees for 30 minutes for each 1/4" thickness. Depending on what I'm curing, I cure between 1-2 hours. I always add extra time for good measure. You can't over cure.
The Kato instructions state to cure for 10 minutes at 300 degrees. Depending on thickness, I usually cure 20-40 minutes. Sometimes an hour. Again, you can't over cure, so I always add extra time.
Great tips from everyone! I also bake all but the thinnest pieces for at least an hour in a pre-heated oven (check your brand of clay and use an oven thermometer to set the right temperature). I sculpt some very large pieces, and with those I often bake the piece in stages, allowing me to add raw clay to the baked clay without distorting the details I've already sculpted. This comes in very handy when adding individual scales to my dragons. I bake the clay covered armature for an hour, then I have something solid to hold on to while adding the tiny scales. I'll generally add about half the scales then do another bake so I don't squish those scales. You can bake repeatedly without any problems with most clays, although lighter colors may darken with subsequent baking. Sometimes I'll put a piece of foil over these to help protect them. These repeated bakings also ensure that the piece is completely cured and very strong.
It is so great to read what other Smooshers do when it comes to curing! I cure multiple times at the minimum time. I use mostly Premo & Fimo, so I cure for 30 minutes at 275. Even on super thin pieces, which is what I have been working on lately. I finish almost all of my pieces with multiple layers of Kato Liquid clay, so each layer gets cured for 20 minutes. And I always pre heat my convection oven. It holds the temperature perfectly, even when it's on for hours.
I have a toaster oven made specifically for polymer clay which has a timer for 30 minutes & markings for temperatures between 230F & 275F, but have put an oven thermometer in it & preheat it for 30 minutes, then bake according to the directions on the clay package, which is usually 30 minutes at 230F. Since I am always experimenting with the clay, it is usually baked more than once. I've never had under baked or burned clay, but have seen what it looks like.
I cannot stress enough to use an oven thermometer. It is important not only to know that your oven is calibrated to the proper temperature but to also make sure the temperature does not spike during the curing process. I do not preheat my convection oven but I know that it takes 10 minutes for it to reach the proper temperature for polymer clay to cure. I add 10 minutes to the cure time to cover this. I leave my pieces in the oven until they completely cool.