So I baked clay on a cookie sheet and I guess you're not supposed to but why...?

[rileykathleen]

And of course AFTER it's already in the oven baking I see not to use surfaces you use for food. Why can't you? Did I just ruin our cookie sheet for anything other than baking clay on or is it still okay if you wash it really well? Nowhere gives a specific reason other than that kitchen stuff and crafting stuff should be separate (which if I had stopped to think about it that makes sense, and I would have covered the cookie sheet). Like, is it toxic or...? I'm kind of freaking out because my mom was just saying how she loves this specific cookie sheet because it was a total steal we've had for 20+ years (it's stainless steel and she got it for $1), and I don't want to have to tell her I was stupid and I ruined it. 

[tiffi_333]

you dont want to mix polymer clay and food items because it can be toxic. Since you used a cookie tray you should be able to wash it really well and it will be ok, but i wouldnt keep using it without laying down foil or something simular beforehand. When you wash it just make sure there are no bits of your clay left because that can be harmful. You can get trays super cheap from thrift stores or some dollar stores so you should invest in your own. When baking on a cookie pan the side touching it tends to get shiny, if its something you would want to avoid you can put a layer of baking soda down and it will protect the clay and dust right off when you're done baking. some people also put a small sheet of batting down which is fine with the low temp the clay bakes at.

if you end up using a pasta machine or something with more crevices they should never be used for food again since it cant be guarunteed that you've cleaned all the clay off. some could sneak down in the crevices then into the food if you cook with them again.

[Diane B.]

The main reason you're probably thinking of is *safety* (for use later with food or cooking). 
That's not really a problem in your case though because your surface is not porous, and there are no little crevices in it where the clay/plasticizer could get trapped and be hard to remove. (So just wash your baking tray with soap and water well or wipe with alcohol till clean.)

Other factors can apply to metal baking trays and polymer clay though when curing especially:
...One is that if polymer clay is baked *in direct contact* with a very smooth surface (metal or aluminum foil, glass, ceramic, etc), it will have a shiny spot in those particular areas after baking.  That's because polymer clay softens slightly when heated but enough to take on the surface texture of anything it's touching.
...The other one is that metals and ceramics, for example, *can* heat up higher than the oven cavity's air temp and therefore darken the bottom of the baking item or just speed up baking from one side.

For both problems, you can just place a sheet of plain white paper on top of the metal tray then place your item on that when ready to bake.  Paper has approximately the same surface texture of baked polymer clay.  You can even accordion-fold the paper for baking beads, pens and some other items.
Other materials will work too under (or over or around) the clay, and especially when you're trying to "protect" the clay from darkening, etc, while baking. 
There's much more info about ways to protect the clay while baking, baking surfaces, safety, and more on the Baking page of my site, if you're interested:
http://glassattic.com/polymer/baking.htm

P.S.  Polymer clay is not "toxic" in the true sense of that word--well, unless you burned it, passed out, and breathed the thick black choking smoke for long enough to kill you.

.

[ilysa]

Polymer clay in non toxic and its not going to kill you if you used the baking tray one time for clay, so don't stress over it. It's best to use a dedicated pan for baking in the future, you can get a cheapy at the dollar store. The concern is like Diane said with pourous things as they can harbor bacteria.