Acid washing blade with heat treat discoloration

[Tango_Delta_Kilo]

Just made my first knife from a .25" leaf spring steel. Anyway I just finished heat treating and tempering the blade. My plan with this is to acid wash in either Ferric or Muriatic acid. There was very little scaling which I have removed already, now leaving the blade a black color. So my question is can I go straight to the acid wash or do I need to resand the blade to remove the black finish from heat treat?

 

[Stacy E. Apelt - Bladesmith]

The "black" layer you are seeing is a layer of steel that has had the carbon depleted. It is called decarb. It is very soft steel and needs to be removed. On the upper bevels, some people leave it to look "rustic" ... but to me, that just looks half done.

A blade in carbon steel is about 90% shaped before HT. After HT it needs to have a few thousandths removed from all surfaces to get rid of the decarb as well as finish shaping the bevels. Pre-HT the edge is somewhere between .030 and .050" thick ( depending on knife size and type). The bevels need to be sanded down to bring the edge to around .010" or thinner before sharpening. At that point it is ready for etching or other final treatments. After handle assembly and all final touch ups, the edge is sharpened as the last step.