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Help Needed: High Temp Salt Composition and Maintenance


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Hi All,
 
I think that I have done my due diligence to search for this information on the forum (and others). Please forgive if I have missed it. I should also mention that I have posted this elsewhere without much luck. Any light you can shed on my questions below would be much appreciated.
 
I’m building a large high temp salt pot kiln which is nearing completion. Despite my searching I haven’t been able to find answers to some questions I have about the salt itself and I hope that some of the seasoned veterans here may be able to help. I also hope that this information may be helpful to others in the future to safely maintain their systems. My questions are as follows:
 
  1. “Neutral” is a term often seen used to describe commercial salts. “Balanced” is another one. I haven’t found much information on what this means except some references to metallic oxides and other oxides building up in the pot which can be cleared by stirring with a graphite carbon rod or other rectification methods. Neutrality is something that seems to get out of whack over time. What I don’t understand yet is how salts can be unbalanced or out of neutral to begin with. Is it simply due to oxide impurities present in the salt to begin with? I’m curious about this because I can’t find a good source for commercial salts and I may be forced to go with a DIY solution. Can I simply rectify my salt bath as soon as I melt my DIY salt? Are there salt sources that are pure enough an commonly available?
  2. For those who DIY their own salts I frequently see a 50/50 mix of sodium and calcium chloride used but I haven’t seen this mix recommended in any official documentation (e.g. Mil-10699). My current plan is to do a class 5 salt (50/50 potassium and sodium chloride) from cheap table salt (non-iodized) and potassium chloride water softening salts. I haven’t found information on the purity of these sources yet, so please share if you know anything. My main question, however, is about the choice of calcium chloride over potassium chloride. Can anyone shed light on the differences/pros and cons of these two options? 
  3.  Rectification is another topic I haven’t been able to find much of information on. I’m trying to find out if stirring with a graphite rod is enough to neutralize my salt or if there are other methods I should be employing (methyl chloride perhaps?) to keep the salt in good condition. Is a carbon rod enough? 
 
Thank you for your help and/or interest. 
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Hi Tyler. I’m afraid I have zero knowledge about salt pots but as no-one else has waded in I thought I might just give a suggestion of my interpretation of (1). I would suspect that neutral would refer to the pH (i.e. pH 7). Acids are corrosive to steel and bases are may also react with various alloying elements. Heat speeds up most if not all reactions so the combination will likely affect the steel quickly. Sorry I can’t be of more help.

"The way we win matters" (Ender Wiggins) Orson Scott Card

 

Nos, qui libertate donati sumus, nes cimus quid constet.

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What do you have for a high temperature salt pot?  You can do serious harm to yourself if it's designed or used wrong.  If you allow the bottom of the pot to heat without a way to relieve pressure they can turn into a volcano and splatter melted salts on you.  Allow some water to get into the salts it will rapidly turn into steam and explode, and again you will be covered in melted high temperature salts.  You could look up Kevin Cashen and drop him an email and he might be able to give you some advice.

 

Doug

HELP...I'm a twenty year old trapped in the body of an old man!!!

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Thanks for the comments so far. 

 

Doug, Thanks for your concerns. They are well founded. I'm also quite concerned about safety and will be taking every precaution and always wearing appropriate PPE. The kiln that I have built is a PID controlled two burner pentagonal electric kiln. I have the ability to crank the top burner up to melt the top first before the bottom. I also will be putting a conical plug in my salt at the end of each HT session and removing it before the next. The vessel that I have made is 3/8in wall stainless steel with a full penetration weld 3/8 in cap at the bottom. The vessel will be tapped and grounded to earth ground.

 

On 10/27/2021 at 5:59 AM, Alan Longmire said:

Have you checked here? http://www.hightemptools.com/salts.html

 

 

Yes I have looked into them. They have some great information in their FAQ section. Unfortunately I'm in Canada so the shipping is horrendous (~$150), but I may go this route if I can't find something closer to home. Based on the operating range of the salt they are selling it should be a class 5 which is just ~50/50 mix of sodium and potassium chloride. If going the DIY rout for mixing my salt is going to be nearly as good I would prefer to just pick it up my salt at the local hardware store and avoid the waiting and shipping expense. Have you ever tried to go the DIY route and mix your own salt? Just curious if you noticed a big difference if you did try it. 

 

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No, I haven't played with salt pots.  I like the idea of them, but I'm not sufficiently productive to warrant the expense. :lol:  Two members whom I know use them don't post much these days, Ric Furrer and J. Arthur Loose.  I've watched Al Pendray use them at a hammer-in, but that's my only firsthand experience with them.

 

Good call on the tapered plugs, those are a very important safety device.  

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Tyler I have been heat treating with salt for about 20 years and it is the only way to heat treat as far as i am concerned.

As pointed out, safety has to be the number one issue, protective gear is a must and serious design consideration of the pot is a must. 

I use KCL for my salt as one of the biggest Potash mines in the country is just down the road 20 miles from where I live.

They produce thousand of tons of the stuff and the product is 99.9 % pure.

Here are a couple of photos of my pot and heat treating some knives our club donated to the Canadian Military in Afghanistan a number of years ago.

If you want some KCL I have two  bags I want to get rid off, send me a pm with your e mail.

 

1024 SALT POT 3.jpg

 

1024 SALT HEAT TREAT ARMY.jpg

 

SALT POT 22.JPG

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Google searches for "Ed Caffrey + Salt Pot" and "Kevin Cashen + Salt pot" will take you to more posts on this subject (on other forums) than you can shake a graphite rod at.

 

The only two posts on this forum I have found that begin to answer the questions you pose are this one:

High temp salt pot build - Tools and Tool Making - Bladesmith's Forum Board (bladesmithsforum.com)

 

And this one: High temp salt pot build - some (informed) questions - Tools and Tool Making - Bladesmith's Forum Board (bladesmithsforum.com)

Edited by Joshua States

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The only bad experience is the one from which you learn nothing.  

 

Josh

http://www.dosgatosdesignsllc.com/#!

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Cal G, That's a good looking pot! Thanks for the offer. PM sent

 

Joshua, Thanks for the searching tips. I have done quite a lot of searching and reading already, but I did come across a few with your key works that I hadn't seen before. Unfortunately, most of the discussion is on building the pot & furnace with not a lot of discussion on the salt itself. On occasion there are comments like "I use a 50/50 mix of calcium chloride and sodium chloride", but these are not followed by a discussion about the reason for that choice, or the maintenance involved for a given mix. As I mentioned above, calcium chloride gets mentioned quite a bit, but in any official HT salt documentation I have read it is an uncommon ingredient and is not used at high percentages when it is (20% max). Perhaps that is because of it's hydroscopic properties? I did fine one post in  my recent search that mentioned rectifying with both boric acid and graphite rods. This is a valuable tidbit, but I would like to know why both boric acid and graphite is required. I understand that the graphite reduces metallic oxides in the mix which precipitates the metals onto the rod, but what is the boric acid doing? Is it combating another form of salt degradation? 

 

I have also done some researching on salt sources. I found that water softener salts are typically very high purity (makes sense for something going into dirking water). They do sometimes have citric acid added in very low percentages (~0.02%), but this would burn off during the first heat with little issue and nothing nasty left behind. Water softener pellets come in either KCl or NaCl version and are about $8/20kg for NaCl, or $35/20kg for KCl. The downside to these of course would be that they are not dehydrated, but a simple baking step would take care of this. I have read some people warn against using DIY mixes, but if I am selective about choosing high purity sources and drying it before use I'm having a hard time seeing a downside. Am I missing something? In contrast to the 50/50 CaCl/NaCl mix, a 50/50 mix of KCl/NaCl is a documented mix with a working range between 700°C and 900°C. 

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  • 5 months later...

I realize this is an old thread, but the reason I use the 50/50 calcium / sodium mix because I can get them at the farm store for less than $20 and they work.

 

I read a thing about the boric a long time ago and it made sense at the time, but I have no idea what it is now. Same with the graphite rod. :lol:

 

I realistically fire these 4-5 times a year at most, but they’re still running absolutely fine 20 years later. I think I effectively changed the high temp salt once, mostly to fish out a dropped blade.

 

I use Brownell Nitreblue for the low temp. Also inexpensive, and carries its own risks.

 

Hope your rig is up and running! :) If I just have one small blade I’ll go by the forge and chase the shadows for fun, but for anything long, or more than one or two, salt is really the way to go.

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