I've just dyed my first parts with a 15psi Pressure Cooker and I made some interesting observations. Yes, I know this thread would probably be better with pictures but I'm having camera issues at the moment and I doubt my cell phone would capture the detail needed.
Always here is the mix I used:
3 gallons of tap water
1/3 cup of non-iodized salt
1 teaspoon of Calsolene Oil
1 TBS of Citric Acid Powder
1 cup of white distilled vinegar
2 packages of black Rit Dye
The cook time at 15psi was about 1.5 hours and in this time the fluid level dropped from about 6 to 5 inches. The parts were contained in a mesh cage
to keep them submerged.
The initial results were very poor. The parts came out a very dark ugly green color. I also noticed that I could see the cage at the bottom of the pressure cooker through several inches of water so it appeared there was very little dye left in solution at this point. So what I did was add 4 more packages of black Rit Dye (to make for 6 total) without adding anything else. I then re-dyed everything for over 3 hours at 15psi and I let the pressure cooker cool over night before opening it this morning. There was about 2 inches of fluid remaining as the top of the cage was just sticking out but the water was a nice opaque black as you couldn't see past the surface. The parts are a very nice rich black but I'm not sure if they are any blacker then they would be had I simply boiled them as I have in the past.
(1) It appears you need ALOT more Rit Dye to get the same rich black. In the past I've obtained a nice rich black with simple boiling and a single bag of Rit Dye.
(2) The cost of a pressure cooker.
(3) There is no easy way to monitor the fluid level during the process. You certainly don't want to run things dry. I'm wondering if I should set the hot plate/pressure cooker on top of a scale as I think that would allow for a way to monitor fluid level. Though this test tells me if one assumes a drop of about an inch per hour they are probably safe.
(1) There is minimal to almost no bleeding of the black after dying. I could rinse the parts in water and the water stayed clear. With simple boiling I can rinse the parts dozens of times and they continue to turn the water black. I'm hoping this is a good sign as with my gear cube kit I glue colored SW&F tiles onto these black parts and with my first puzzle my attempts to seal the puzzle resulted in some black bleeding into the colors parts. So I think this is a very good sign.
The black may go deeper into the parts with pressure cooking but without cutting the parts I can't tell. To me it appears the higher temperature obtained with pressure cooking actually breaks down the dye while its still in solution. I was thinking I'd read this observation by someone else before and I did a search of Brandon's post as I thought it was something he had said but I couldn't find it. Even the 2 inches of solution that was nice and opaque didn't seem to have the staining power that the solution left after boiling had. You invariably get a few drops on your hands or a table surface and it wipes up without making a permanent black spot. What you get on your hands washes off with a little soap and water. So in some sense this made clean up a little easier. Still I used a hot plate and did this in my garage... I would NOT recommend you do this in your wife's kitchen. LOL!!!
I know Brandon uses a pressure cooker, not sure anyone else does but I'd be curious if my observations were comparable to others that have tried this. I'm now also curious if its possible to buy Black Rit dye power in something other then those 1-1/8 oz boxes. If I stick with this method it seems I may want to buy the stuff by the gallon.