How Do I prevent Scale Build up When Heat Treating a Knife?
Well making my third Damascus knife 13 layers ,when i get it polished and to the point of tempering when i put it in the gas kiln red hot to loose magnetic it warped every which way .made out of all the same coil spring steel and hammer welded plus all sorts of slag all over it now so while it was red hot gently hammered it straight again and annealed . so how can i get it polished perfect and temper with out slag and distortion next time now after i sand and polish it again ? whats to keep it from happening the next time I go to temper it ???
This is a little tricky to answer as I have not seen all the steps that you have gone through.
Slow cooling (annealing) in vermiculite or wood ash to relieve the stresses is very important. This should remove much of the tension between the two different types of steel. This should help reduce the warpage when you heat for quenching.
If you are heating the blade in a gas forge hold it so that the back is up and it heats evenly from both sides this should help with the distortion as well. It should only be heated just past the magnetic point. It doesn't have to get super hot. Then pull the blade out and test with a magnet until it just pulls then quench the whole thing in the appropriated quench medium. Water or oil. Then temper evenly 425 degrees in a toaster oven. Then a quick sanding and hold the edge in a water bath (about 1/4 inch deep) and paint the temper colors on the back with a torch. Blue on the back straw on the edge.
Then sand to final finish always keeping the edge and the blade cold by dipping in water.
In short I rough grind and sand to about 80 grit before I heat treat. The final sanding is done after the blade is hardened. When I harden there will be some scale formation but it is really very little as it is only up to color for about a minute and not very hot. Just enough to lose the magnetism.
I know some people use a nickel foil to wrap around their blades so that it doesn't oxidize very much. I have not used this technique but understand it works well. You would have to source a knifemaking supplier for the nickel foil.
Hope this helps.