Repoussé is a traditional metalworking technique where a raised or relief surface is created on the front of the metal by hammering a design from the back of the metal. It is a french term whose origins mean to push from behind. It is often used along with another traditional technique called "chasing" where the design is then sharpened or refined from the front by sinking in the metal on that side. The effect of repoussé and chasing creates what is often referred to as embossing. You are dealing with the elastic qualities of a malleable metal, so there is no actual loss of metal.
This is a relatively simple technique to replicate in metal clay while the material is in its clay state. It is a great way to incorporate original designs and textures, even for the beginner artist. You will need something to create your impression with, the pieces in this post were done using scratch foam, so that is the process I will focus on here. Scratch foam is a popular and economic material used in basic printmaking. If you think it is an elementary school material, check out the sophisticated prints by artist Annette Mitchell who is able to get woodcut-like imagery from recycled meat trays. If you can not find it at a local craft store, you can always use cleaned foam trays from the supermarket. Just cut yourself a square to work with.
Begin by working out a simple design. I did a series of pieces based on swirling cloud patterns, they all began with the same frame, but I played with the interior a little differently on each piece. I like working out designs like this on tracing paper so that I can quickly replicate elements I am happy with. I then tape the tracing paper to a piece of scratch foam and lightly go over it with a ball point pen, just enough to get a slight indent in the surface. I then use an embossing tool to go over the lines, but maintain medium to light pressure. Each time the lines will become more pronounced, but I don't end up ripping or wrinkling the foam. As the lines get deeper, I change the gauge of the embossing tool if I want thicker lines. The key is patience until you get a feel for the right pressure on the foam. If you are looking simply to create texture sheets to emboss with, you can use anything you can dent the surface with from a comb to a meat tenderizer. Keep in mind the scratch foam has a more "smooth" side and a more "rough" side, you will need to look closely at the foam to discern this, I recommend carving into the smooth side. In any case, the surface of the clay will pick up the surface of the foam, which is more subtle on one side than the other.
Once the design is carved in the foam, I lube the foam so the clay will eventually release. I set my rolling slats (or cards) on top of the foam on either side of the design a little thicker than I want the piece to account for the raised areas. If the piece of foam is too small to support the width of the slats, I lay additional slats (or cards) next to the foam to keep them even. I then roll my clay into the design the same way I would roll it onto any other texture plate. I then gently pull it off and trim the excess. Once dry, they are fired and finished the same as any other piece. The bonus of the scratch foam is that it is relatively inexpensive and fairly forgiving.
If you chose the wrong side and this impacts your design (which you will be able to see as soon as you pull it off) simply smooth your metal clay piece with a damp sponge - as long as you are light with your pressure it will clean up the surface without removing the embossing. Also, remember this is a printmaking process - you are essentially creating a "stamp" that you are embossing your piece with so your design must be the REVERSE of what you plan. In a symmetrical design this does not matter, but in the design of a piece like I did here it can make a difference. I planned it backwards because I have done this enough times to think about it that way. If you are not quite there yet, this is another good reason to workout designs on tracing paper, because you can then lay them on the foam on either side. If you draw your design exactly the way you want it, you should then lay the tracing paper face down on the foam. It will reverse the design on the foam and come out the way you intend in the clay. Since it's tracing paper, you can see through it and can still easily trace it.