Forge - holding the steel in a small forge at an exact temperature for 10 minutes without heating beyond 1425F ,eg., is difficult. Adding a PID controller to the forge and using a muffle could help.
Quenchant - the quench speed is everything in attaining a hamon. Using any oil will decrease the activity, Using vegetable oil in lieu of a fast commercial quenchant ( like Parks #50) will further decrease the results ( or virtually eliminate them).
Try to learn the quench using brine. Once you have it just about mastered....you will probably go back to fast oil.
Steel - the steel needs to be as shallow hardening as possible. Little or no alloy content. .70-1.00% carbon, and .30 or less Manganese is the perfect steel, but such a mix is hard to find. The usual suspects are 1070-1095, W-1, and W-2.
Alloy steels like 5160,52100, (and, obviously, stainless steels) will not work.
Clay - coat the entire blade with a thin wash coat, and coat the shielded areas with an 1/8" coat. Measure the blade at the spine before you coat it. Measure it again after the coating. You would be surprised how often it is a lot more than .250 wider.
Condition of the steel - any blade being quenched needs to be ready for the stress it will be under, but shallow hardening steels need it the most. Properly normalize/stress relieve the blade before the clay coating. Sand the surface smooth and remove every grind mark. Eliminate any possible stress risers, such as sharp corners or edges. Leave plenty of edge meat,about .040", and round the edge. Round off all edges a tad ( many folks grind the edge and spine to a prefect flat surface, which creates a sharp 90 degree angle with the flat sides).
The quench - If using oil,after the plunge, hold the blade still for about 2 seconds, to allow the edge to cool below the pearlite nose, and then move it back and forth in a cutting motion. After about 5-7 seconds, pull it out and check for warps. Fix immediately and return to the quench tank to cool the rest of the way ( all adjusting of shape needs to be done before the blade cools to 500F). Don't remove it from the warm oil for at least two or three minutes. Gently clean off any clay still stuck on ,and temper at 300 immediately.
Resist the temptation to give it a quick grind and dip in the FC to see the results. Cracks can be forming while you do that if the steel isn't snap tempered quickly after quench.
If doing a water quench you will have to work out your own in-out timing ( I use, IN-1-2-3-OUT-1-2-3-Back in). The immediate post quench temper still applies.
