To make the most of your clay, it's important that you don't use too much so that your jewellery weighs a ton and you use too much in one piece. When you're learning, you tend to roll clay out much thicker than you need - this is of course fine with copper and bronze but expensive for silver!
Traditionally, metal clayers would put clay on a non-stick surface but before rolling out would place playing cards (normal playing cards) either side of the clay that the roller would rest on. The height of the playing cards would prevent how deep the clay was rolled out and would ensure that the clay was an even depth all over. Some people still use playing cards but I find they slip and slide around (even if I have stapled them together!) so I have invested in special spacer bars for metal clay. They are relatively inexpensive at around £5 for a set but they last for a very very long time.
The typical "depth" of clay that you need for most designs in silver is around 4 playing cards thick which equates to 1mm. However accent pieces can go as low as 0.75mm or even 0.50mm. At the other end of the scale, some designs, perhaps where a deep imprint is required or where you need to surrrond a bezel, require you to roll out to 8 cards / 2mm or even 12 cards / 3mm thick so the larger spacers are definitely NOT redundant!
Copper and bronze clay does need to be rolled out a little thicker than silver initially. I'm not sure why this is but from my experience it does! If I'm using a texture plate on copper or bronze, I may start at 6 cards thick. Of course you also need to adjust how thick you roll out any clay to adapt to a texture plate if you're using one.
Not all texture plates are the same. Some are quite deep so you need to roll your clay out much thicker.
In the article I completed for Beads & Beyond Magazine, the texture plate I used was deep so I initially rolled out the copper to 1.5mm (6 playing cards). Then I removed those spacer bars and replaced them with 1mm (4 playing cards), placed the texture mat over the clay and rolled again. By having the 1mm spacers next to the clay, it avoided the clay becoming too thin and limited the depth of the impression. If I hadn't put any spacers there, I could have rolled too hard or unevenly and ended up with thinner and thicker areas of clay and even holes!
If I had used a texture plate with a more shallow design (so think wallpaper or a leaf or something like that) I may well have rolled out to 1mm (4 playing cards) and would potentially have left the 1mm spacers in place.
Just for reference here's how playing cards add up to mm:
3 cards = 0.75mm
4 cards = 1mm
6 cards = 1.5mm
8 cards = 2mm
12 cards = 3mm
Spacer bars (unfortunately) are not colour co-ordinated the same by all manufacturers so a white spacer bar from one company may be 1mm but from others it may be different. It's a good idea if you use spacer bars (I love mine) to write on the top of them what depth they are and the amount of playing cards they equate to. I use a Sharpie so it's semi permanent. You can see from my photo below that I've bought different sets from manufacturers and several of the colours are the same but the depths are completely different! Simply picking up an orange or grey one (if it wasn't labelled) could lead to some odd results! By the way, the black one is 0.75 / 3 cards but you probably can't see the writing on that one. If you buy a set from www.metalclay.co.uk, you'll receive a pack similar to the second smaller photo and you can see how to use them in the photo after that.
Above are examples of two texture plates that demonstrate the different depths that can be used with metal clay.