I had a fellow ceramic artist ask me about the Van Gilder's flashing slip. Here's my understanding of it. The information presented is from my own experiences.
The Van Gilder's flashing slip is a thick slip that's available for use at the Irvine Fine Arts Center. It can be used thick for slip trailing or thinned down for painting application with a brush.
In theory, it is supposed to "flash" (ie have twinges/flashes of yellows, oranges) when you fire to Cone 10 reduction (highly reduced firing). I've only seen in flash orange once on my pinch bowls and it was rather pretty.
I use it because it cracks predictably at Cone 10. It starts cracking after bisque fire to give you a nice texture. Most of the time, I use a wash of red iron oxide or manganese dioxide to impart color into the cracks. The red iron oxide will be anywhere from reddish brown to warm black. The manganese dioxide is blue black. I sponge off the excess stain from the cracked surface leaving only the stains in the cracks. The piece is then fired to Cone 10 reduction (in a gas kiln).
When applied correctly, the results are very attractive. This piece has the flashing slip applied to the tentacled pods and the short stubby thorns. I used manganese dioxide stain. You can see the cracking in those areas.
This is the kind of flashing that can happen in the correct environment. Notice the oranges.