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What's the mixing ratio? Any hardness measurements on the surface to to check for decarb after hardening?There are several ways. One is to make a paste of boric acid and alcohol, apply it to the blade everywhere you don't want oxidation. Heat as normal and quench as normal.
Another is to pack the blade in a stainless foil pouch with a scrap of paper inside the bag with the knife blade. The paper will burn, consuming all the free oxygen in the pouch leaving you with no oxidation on the blade, but you need to work quickly when it's time to quench to cut the pouch open so you can get the blade into the quenchant sans pouch.
Yet another is to use either a vacuum furnace or alternatively an argon purge in your furnace while it's heating the blade.
All of these work, all have their plusses and minuses. I'd try the boric acid paste first.
Just to make it clear - coatings and foil packets will reduce decarb, but not totally eliminate it. Additionally, the edge should be left fairly thick pre-HT and then the final bevels ground after tempering. Finally, the blade will need finish sanding to bring it to its final surface look.
So, short answer, - You should always re-grind the blade post HT.
If making a boric acid coating, this is a better formula that the saturated solution mentioned:
Mix 8 ounces yellow ochre, 6 ounces boric acid, 3 ounces satanite, and 1 ounce gum tragacanthin. then add enough denatured alcohol to make a medium thick paint. Paint on the blade and let dry before putting in the forge. You can play with the ingredients ratios if you want, but this is what I have found works well. This is similar to Turco, which is a very good commercial anti-decarb product, but pricy.
NOTE - only use boric acid coatings for basic carbon steel HT done below 1600°F. At higher temperatures it becomes corrosive and will damage the blade surface.
Ways to reduce decarb:
The number one way to reduce decarb is to use a HT oven. A forge is just about the worst place to do HT as far as decarb goes. That doesn't stop many folks ( including me from time to time) from using the forge to do HT in. You just have to grind a few thousandths more away post-HT.
Salt pots. These come as close as possible to no decarb. These are not amateur devices and are expensive.
Sand pots, safer than salt, but more expensive.
Vacuum ovens - expensive commercial machines.
Argon purge ovens, they work fairly good reducing decarb, but argon is an added expense. (Some folks use nitrogen)
Stainless foil packets - these are what most bladesmiths use. The cost is fairly low per-knife and use is simple. They can only be used with a HT oven.
Blade coatings - Turco, ATP, Brownell's, satanite/clay, and many others all work fairly well. Using one of these is important if you HT in a forge.
Does this mixture work better than heating (around 350F) the blade to melt boric acid to form the "glass" layer over the material?Mix 8 ounces yellow ochre, 6 ounces boric acid, 3 ounces satanite, and 1 ounce gum tragacanthin. then add enough denatured alcohol to make a medium thick paint. Paint on the blade and let dry before putting in the forge