My dyeing experiment was a qualified success. I didn't want to go to the trouble of doing a test piece first, so I just hacked off a couple yards of velveteen and went to it. I used a big cast iron dutch oven that I'd acquired some time ago. I didn't really care if the iron affected the chemistry of the dye since I was essentially going to be using an iron oxide pigment anyway. I used a couple ounces of alum and an ounce of cream of tartar as a mordant, though it may not really have been necessary. I decided to do it anyway just in case. I boiled the cloth in the mordant for an hour, then let it sit and cool for another couple hours. After rinsing, the velveteen had taken on a very slight pink hue, though I don't have any idea where it came from.
Next I prepared my dye bath. I had procured a couple pounds of freshly excavated clay from a local construction site. I cut the top off a milk jug and mixed the clay with some water until I got a smooth consistency. I used an old piece of window screen and some duct tape to build a strainer to keep the roots and rocks out of the pot. I boiled the cloth in the dye bath for an hour or so, then decided that it probably wasn't going to get any darker. Now came the hard part: rinsing. There was a lot of silt in the pot, and I ended up holding the cloth up in the shower to get it all rinsed out. I wonder if I could have gotten the same color with less actual clay in the dye bath.
In the end the color wasn't exactly what I'd hoped for, but I'm still quite happy with it. I was aiming for a dark orange-red, and I got a bright rusty orange color. Considering how much material I had bunched up in the pot, the color was surprisingly even. I also worried that the abrasiveness of the clay particles would damage the material, but it seemed to have no ill effect. All told, I spent about $3 for mordants which I may not have needed, and everything else came out of the ground. I've tried to get the final color right in the pictures.