About a year ago, I saw pictures of Wolfgang's Mordheim table http://gidian-gelaende.de/03c19899280940b01/03c19899c4139102b/03c198995a14eed01/index.php, and was sufficiently impressed that I tried my hand at etching (using a dull pencil) cobblestone patterns into foam. Although successful, it was a labor intensive operation.
A few months ago, it struck me that a faster way to create a cobblestone pattern in extruded polystyrene (aka blue board, pink board, Selitac, Depron, and/or Cellfoam 88), was to make a stamp and stamp the pattern into the foam.
So I started playing around with making stamps. Some methods were successful, many were not.
Caveat: this is my current method ... I don't envision changing it much, but maybe someone may have some good ideas:
1. Pro-Create Sculpting Putty (about 1/4" - 1/2" of the strip). You could use GW's Green Stuff, but Pro-Create seems to dry a bit harder. (about $1-$2 worth)
2. A 1 1/2" fender washer (about $0.20).
3. Scuplting tools (although in a pinch you could use a toothpick with a flattened end).
4. Some scrap paper.
5. Some water.
6. Some sand paper.
7. A hobby knife.
1. Mix up the putty (mix a little less than you think you'd need ... since it always seems to take less than you think you need).
2. Flatten the putty onto the fender washer (curl it up over the sides so the washer won't fall out after the putty dries).
3. Set the putty/washer face up on your scrap paper (it'll stick to any surface you put it on ... so make sure something you don't care about is underneath it).
4. Use your sculpting tool to push a cobblestone pattern into the putty. Start from the center and work your way out. Push straight down and try to leave an edge about 1mm (or less) wide between cobblestones. The putty should start to look like the cells of a beehive. Also try to keep the walls of the cells vertical and all at roughly same height (about 1/4").
5. When you reach the sides of the stamp, scuplt the cobblestones in such a way that they peter out (and run right off the side of the stamp).
6. Important: you need to scuplt the whole surface of the stamp, any high areas left will mean compressed areas on the foam. So try to keep the joins between cobblestones crisp.
7. Let it dry for a day or so (don't take it off the scrap paper).
8. Peel the scrap paper off (some will be left behind, but don't worry about it).
9. Take the sandpaper, and sand it down until the entire pattern is level. This will make the pattern less "rough" (i.e. more even) when you stamp it into the foam. This is particularly important for patterns with smaller cobblestones.
10. Optional: Take a hobby knife and cut/whittle any thick cells walls down a bit.
I've only done test pieces, but so far it seems to work.
1. Take the palm of your hand, and press the stamp into the foam. Don't press too hard (or the foam cracks). The smaller the cobblestone pattern, the harder you need to press. I found you want to listen for it to just start making a cracking noise.
2. Move the stamp, rotate it slightly. Do it again. You can overlap the stamping slightly. Practice this on a test piece until you get the hang of it.
3. You may need to go back over and connect some of the "stampings" with a dull pencil (although I don't think it's 100% necessary).
I'm also going to try "roughing" up the surface of the foam, by pressing a rock into it, prior to stamping. I hope that this gives the cobblestones more texture. I haven't tried this yet.
As far as speed goes, once I get all the ingredients together, it takes about 15-20 minutes to make a stamp (not including drying time). I can stamp out a 8" by 8" section of foam in about 2 minutes (not including any time to "fix" it up with a dull pencil). Overall, it's a little faster than the 4 hours to do a 2' by 2' section which is what I remember it taking me to do before (should now take about 20 minutes). So far, it seems worth the time to make a stamp.