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Hi Kevin,

Could you elaborate on the 400F interrupt please?

Thank you,

Beverly Devereux

Hello Beverly, for simple carbon steels with basic carbon solution the Ms point is around the 400F mark. Ms is the point at which martensite actually begins to form, in other words when the blade actually begins to harden, so long as cooling is continuous martensite is the phase you will get in transformation. By cooling all the way from the heat at which you came from the oven/forge to 400F you will avoid all the undesirable phases that will give you issues with edge performance, but at Ms or below the drastic cooling is no longer necessary so you can save the blade from needless stress by interrupting. Once in the air you can also sight down the blade and straighten any warps before it fully hardens, this is also the time that I tap the clay off the sides of the blade. The clay retards the cooling of the liquid enough to permit the formation if some of those undesirable phases, mostly pearlite at around 1000F, but this is a separate process entirely from the Ms point when the edge that is not shielded by clay will convert all that is left to martensite. The interrupt will also allow for equalization in temperatures which would cause phase transitions to get out of synch and create distortion because of the clay. Oil is notorious for causing the tip to drop in a negative curvature (reverse sori) when the blade is clayed, an issue not as prominent in water because the cooling is rapid enough not to seperate the transformation as much.

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Hello Beverly, for simple carbon steels with basic carbon solution the Ms point is around the 400F mark. Ms is the point at which martensite actually begins to form, in other words when the blade actually begins to harden, so long as cooling is continuous martensite is the phase you will get in transformation. By cooling all the way from the heat at which you came from the oven/forge to 400F you will avoid all the undesirable phases that will give you issues with edge performance, but at Ms or below the drastic cooling is no longer necessary so you can save the blade from needless stress by interrupting. Once in the air you can also sight down the blade and straighten any warps before it fully hardens, this is also the time that I tap the clay off the sides of the blade. The clay retards the cooling of the liquid enough to permit the formation if some of those undesirable phases, mostly pearlite at around 1000F, but this is a separate process entirely from the Ms point when the edge that is not shielded by clay will convert all that is left to martensite. The interrupt will also allow for equalization in temperatures which would cause phase transitions to get out of synch and create distortion because of the clay. Oil is notorious for causing the tip to drop in a negative curvature (reverse sori) when the blade is clayed, an issue not as prominent in water because the cooling is rapid enough not to seperate the transformation as much.

I know this is an old thread, but I've recently ordered some Parks 50 for the first time and for the knife I'm making currently (1095 kitchen/chef knife) I am hoping to produce a visible hamon so I'm perusing related posts. Kevin, as I read your instructions my question is how do you know when you've reached the 400F mark and it's time to remove from the oil to air cool? Thanks!

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I know this is an old thread, but I've recently ordered some Parks 50 for the first time and for the knife I'm making currently (1095 kitchen/chef knife) I am hoping to produce a visible hamon so I'm perusing related posts. Kevin, as I read your instructions my question is how do you know when you've reached the 400F mark and it's time to remove from the oil to air cool? Thanks!

I would be interested in knowing if the 400F interrupt happened in a oven after the 10 seconds in the parks 50?

Sorry, I got busy and the questions in this thread slipped past me. Most oils have a vapor point at around 400°F and so there are signs to look for. If the blade is still wet with oil but has very light, wispy vapors coming off, it is about right. If it looks dry or is smoking heavily it is too hot, and if it is wet with no signs of heat it is too cold. With Parks #50 this normally equates to about a 9-10 second count with moderate agitation, other oils need to be worked out for their time. The interrupt is in air, oven cooling would be VERY bad.

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Sorry, I got busy and the questions in this thread slipped past me. Most oils have a vapor point at around 400°F and so there are signs to look for. If the blade is still wet with oil but has very light, wispy vapors coming off, it is about right. If it looks dry or is smoking heavily it is too hot, and if it is wet with no signs of heat it is too cold. With Parks #50 this normally equates to about a 9-10 second count with moderate agitation, other oils need to be worked out for their time. The interrupt is in air, oven cooling would be VERY bad.

No problem at all. Thank you for the explanation, that makes sense. I'll try the interrupted quench with my next hamon. This was my very first attempt at a hamon, and my first kitchen knife for that matter. It was a Christmas gift for my wife, forged from 1095. The photo is not great, but I was able to obtain a hamon using satanite as the clay, quenched without interruption in Parks 50. It was etched with Ferric Chloride. I was hoping for a bit more contrast, though as I understand it, traditional hamons were more subtle than what bladesmiths often strive for these days. Thanks for the help!