Ive gotten into hamon work lately. WHen I make a blade without one, I want it to have one. For whatever reason some love them, some dont....to me they are an elusive aesthetic trait that a knife can have that at the same time reveals the real nature of heat treating.
While i am no expert, my method is as follows:
-Finish blade to a rough grit to give the clay something to stick to
-apply clay with a pattern on the top 1/3 of a blade. Any patterning I do is mirrored on both sides.
-Realize that the pattern you apply will not exactly follow the hamon it will produce. You are moving around heat, not painting the hamon.
-Heat the blade lightly to cure the clay, let it sit overnight under a hot shop light or cure it with a torch. If the clay is wet, it will often pop off in the forge or upon entry into the quench before it does its job.
-heat the whole blade normally, making sure that clay gets up to temp. Its not insulating the blade from heat. The clay will be glowing red just like the steel when its ready for quench.
-Plunge the whole thing into the quench or you can try an edge quench.
-With 1095, you have a fraction of a second to get the steel below the nose. The clay serves as a bigger mass of heat and at this point does insuate that steel from cooling rapidly enough to achieve full hardness. The steel will harden in the non coated areas normally, while a general line will develop near the clay edge where the steel is unable to harden effectively.
-Scrape off any clay that did not come off in the quench and then finish the blade just like any other blade.
-Polishing a hamon is a whole other monster. Getting the subtle wisps and details is a process in patience, etching, polishing, etc.
When it comes down to it, its all in the HT. Some hamons are crazy and are done with no clay but very precise and accurate heat treating. I have yet to achieve a wavy, wild hamon without clay, but I am working on it!