The substrate in the tank is Schultz Aquatic Plant Soil, in its natural color. The substrate in the tray is the same product, dyed red by me.
Last night in my fatigued-addled state, I said I might have figured out how Turface does it. Well, there are more things in heaven and earth than I will ever know about, so I can't really claim that.
But after much searching for any method that will permanently color calcined clay after firing, I only found one candidate.
That method is concrete acid stain
. A chemical is applied, which with the help of an acid, reacts with the existing minerals to create an inert permanent color.
There are commercial products available, but I found a website describing how to make your own:
I had one of the chemicals on hand, ferric chloride. It's not an optimal stain, because it significantly eats away at the surface; which removes a lot of the stain it produces. And it's also rather nasty. But it's good enough for a test.
I placed a bit of Schultz in a disposable plastic cup, and added enough ferric chloride to cover it. Noticeable fizzing occurred. I left it for 15 minutes.
Then I added some water, and slowly started adding baking soda to neutralize the ferric chloride; stirring with a disposable plastic spoon. More fizzing with each addition. Once the fizzing had completely stopped, I strained out the Schultz and discarded the now inert solution.
The Schultz then went back in the cup, with water and more baking soda, to sit overnight; in order to neutralize anything still in the porous clay.
Finally, I rinsed it until the water ran clear.
The result is exactly what I expected. After rinsing, I soaked it in water for a while; the stain appears to be permanent, and doesn't bleed out. Since the stain is iron-based, I kind of wonder if this sample would now function like Flourite.
If you look at the DIY stain website I linked, excluding the green copper-based stain, the basic colors available correspond to the colors of commercially colored calcined clay products. Interesting.
So what good is this? Probably none. I really have no intention of staining a bunch of substrate and filling an aquarium with it, especially given the chemicals involved.
But it can be done. And I've done it. That's satisfaction enough for me.
Oh, and definitely don't bother with Rit dye. Though you can get some interesting colors, like this lovely shade of indigo:
It will never, ever stop bleeding color; no matter how you try to fix the dye.