Has anyone here ever done that?
Oh definitely! In fact, there's a whole sub-category at my site that has suggestions for dealing with 'burnt" clay on the Baking page
> Remedies for Burned Items
You do want to be careful about the unhealthful smoke and fumes that happen when plastic burns though (for polymer clay in general, that happens at about 385 F and eventually the clay will get bubbly too).
I have a small package of brown scuply . . .those are pretty thick pieces, I says-so...then move it to the lowest rack and turn the oven to 300 and stalk off for a while. . . The pieces aren't brittle at all, just black.
Even if your oven was at the temp you thought it was (often not the case), and the clay was not placed in a hot spot or too close to walls or coils or on certain types of baking surfaces, the 3 main "Sculpey" clays** are worse for darkening problems than all the others--and they do it even at lower temps than recommended by the manufacturers. So 300 is way too hot for any brand or line of polymer clay except Kato Polyclay (though the temp can be upped briefly do clear up liquid clay and do a few other things--but still not on those Sculpeys).
As for the brittleness, the more any polymer clay polymerizes as it cures (which is what makes it harden), the stronger it will be.
So going all the way to burned will have polymerized it a lot presumably, and even the "weaker" lines of polymer clay like those Sculpeys will be stronger if they're baked longer; in fact, those sculptors who just like to completely paint over polymer clay will often bake SuperSculpey for a few hrs just to make it stronger, and the color won't matter in that case since they'll be painting over it.
Those Sculpeys are also more brittle in any thin or projecting
areas than the other brands/lines of polymer clay, though their surface will "feel" a little harder. Thick and rounded shapes though are naturally strong.
and even worse original (white or terracotta, boxed) Sculpey
Painting clay-can you paint it before you bake? See, I like having the black clay base to add stuff to, and painting it black afterwards around tiny things would be hard. I suppose I could buy black clay...but surely there's a way to color it first.. . .Maybe I'll burn it outside
You can color polymer clay in various ways.
Generally, "painting" is done after baking but in some cases can be done before when using various colorants (not usually the regular paint for clay which is acrylic paint).
Instead of burning the clay intentionally though to get the black color, you'd be better off to just start with purchased black clay which comes in any of the brands/lines of polymer clay.
There are also some "black" powdered pigments that you could use on or in the other colors of polymer clay but starting with black clay would be much easier.
You could also add another colorant like artists' oil paint but you'd need too much acrylic paint to make a nice black and too much of anything that contains water isn't good to mix into
raw polymer clay.
If you'd like to use a lot of black clay, you might want to just order larger "bricks" of it, which will often be cheaper and/or buy it online.
If you want to check out some of the online suppliers to see what their prices are, check out this page of my site under Mail Order