Editing Differential heat treatment (section)

====Insulation coatings====
A differentially hardened blade will usually be coated with an insulating layer, like clay, but leaving the edge exposed. When it is heated to red-hot and quenched, the edge cools quickly, becoming very hard, but the rest cools slowly, becoming much softer.<ref name="autogenerated114">''Knife Talk II: The High Performance Blade'' By Ed Fowler – Krause Publications 2003 pp. 114–115</ref><ref name="pbs">[https://www.pbs.org/wgbh/nova/ancient/secrets-samurai-sword.html NOVA | Secrets of the Samurai Sword]. Pbs.org. Retrieved on 2012-11-11.</ref> The insulation layer is quite often a mixture of clays, ashes, polishing stone powder, and salts, which protects the back of the blade from cooling very quickly when quenched.<ref name="samuraisword1">[http://www.samuraisword.com/REFERENCE/making/japanse_swordmaking_process.htm Japanse Swordmaking Process ~ www.samuraisword.com] {{Webarchive|url=https://web.archive.org/web/20090923131554/http://www.samuraisword.com/REFERENCE/making/japanse_swordmaking_process.htm |date=2009-09-23 }}. Samuraisword.com. Retrieved on 2012-11-11.</ref><ref name="autogenerated1960">Smith, p. 49</ref> The clay is often applied by painting it on, coating the blade very thickly around the center and spine, but leaving the edge exposed. This allows the edge to cool very quickly, turning it into a very hard [[microstructure]] called [[martensite]], but causes the rest of the blade to cool slowly, turning it into a soft microstructure called [[pearlite]]. This produces an edge that is exceptionally hard and brittle, but is backed-up by softer, tougher metal. The edge, however, will usually be too hard, so after quenching the entire blade is usually tempered to around {{convert|400|F|C}} for a short time, to bring the hardness of the edge down to around HRc60 on the [[Rockwell hardness]] scale.<ref name="autogenerated114"/>

The exact composition of the clay mixture, the thickness of the coating, and even the temperature of the water were often closely guarded secrets of the various bladesmithing schools.<ref name="autogenerated1960"/> With the clay mixture, the main goal was to find a mixture that would withstand high temperatures and adhere to the blade without shrinking, cracking, or peeling as it dried. Sometimes the back of the blade was coated with clay, leaving the edge exposed. Other times the entire blade was coated and then the clay was cut away from the edge. Another method was to apply the clay thickly at the back but thinly at the edge, providing a lesser amount of insulation. By controlling the thickness of the edge-coating along with the temperature of the water, the cooling rate of each part of the blade can be controlled to produce the proper hardness upon quenching without the need for further tempering.<ref name="samuraisword1"/><ref name="autogenerated1960"/>