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Rigidizing sequence check plz


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I read a post from Frosty that was very very detailed in the application of inswool and rigidizer but i still have a couple questions. 
 

does the wool need to be saturated with rigidizer or just enough to get some good penetration?

 

since I’m using two layers of 1” wool, for the first layer, the one that touches the forge shell, does it need to be rigidized on both sides (the inner surface that will touch the second layer of wool, as well as rigidizing the outer surface that comes in contact with the shell)?

 

and finally, I’m seeing a lot of different stuff on curing it. From what i can gather, i shoul wait for it to air dry, covered and moist for 2 days, then fire up the burner until the wool looks red (and this makes it harden)?? Is that correct? 
 

When all that is done.. then proceed to the kastolite. I have read that since it will all be encapsulated in kastolite/metrikote, that rigidizing may not be necessary.. however a jar of fumed silica is 12$ on Amazon. Compared to the price of all the other materials and the health risks involved.. it seems silly to me to skip this step even if it might not be absolutely necessary.. it’s cheaper than a takeout meal :)

Edited by Nicholai
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Rigidizer is not essential.  If  you cut your ceramic matting to size it will stay where you put it.  I would give the matting a light spritz of water and cover it with a castable refractory.  I would not recommend that he ceramic matting be exposed directly to flame.  It will break down over time and expose you to the fibers which can cause silicosis.

 

Doug

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What Doug said, except don't be paranoid about silicosis.  Numerous studies have proven ceramic fiber blanket is as safe as any dust, i.e. don't breathe any dust if you can help it.

 

My instructions for rigidizer are as follows: 

 

1. Don't buy it.

2. If you have it, don't bother with it, it's not for what we do.

 

It's for big walk-in furnaces where there is no chance of wall abrasion.  It actually reduces the insulating abilities of the wool.  

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I’m not really trying to argue with anyone, but almost all of the publication I’ve read about the fibers at high temp being innocuous, were from the 80,s and 90’s. I have also read more recent papers from 2010 outright claiming that cristobalite is a know carcinogen. 
 

this is what i was getting at in my first post…for 12$!!!!! .. why not just do it?? In the grand scheme of the forge, it won’t really hurt the forge. I just wanted to know what the process for drying it was, and how much soaking in it (rigidizer) needed?

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Understood, but as long as the fiber is sealed under the refractory it's not an issue.  Safety is always a good thing to think about.  

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1 hour ago, Alan Longmire said:

Understood, but as long as the fiber is sealed under the refractory it's not an issue.  Safety is always a good thing to think about.  

Yeah that’s what i was thinking, but then the paranoia got to me. 

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Posted (edited)
15 hours ago, Alan Longmire said:

Understood, but as long as the fiber is sealed under the refractory it's not an issue.  Safety is always a good thing to think about.  

Just did a few hours of reading and researching and here’s the best i can find. I looked at old papers , modern papers, new papers, old studies (rats were harmed in the making of those studies ), new studies in summary: rats died. And thus cristobalite crystals that come from refractory wool generally heater heater than 2000 degrees has been deemed as a carcinogen by few organizations and deemed as an agent that may potentially cause cancer or a potential carcinogen based on evidence from other testing.   
 

ADDITIONALLY.. i read through all the MSDS for ceramic wool, rigidizer (fumed silica etc.) and for cristobalite. The consensus was … there was NEVER.. NEVER!!!!!!! any mention of rigidizing the wool. Believe me i looked. Hard. Trying to find something telling me that the approved practice was to rigidize the wool first. But i found nothing. I’m not sure rigidizing locks in the fibers any more than say.. glue.. or hair spray can. Because once it comes up to temp, it’s my guess that the fibers r blowing again. The only purpose i found for rigidizer was to help shape the wool and help hold its shape after drying. 

 

ceramic wool: bad to work with at normal temps. Skin, eye, lung irritant. Unless you’re rolling wool sheets everyday without safety gear, you should be fine long term, but potentially suffer short term effects.. itching, runny eyes, coughing. .. none of these sound appealing to me. My take on it is that i am wearing eye, nose amd skin protection even if it’s just to stuff my forge with wool. And if I’m working with it on a daily basis.. full PPE all round.  
 

Final note.. me building my forge .. I’m protecting eyes nose and skin when handling the wool. And based on actual evidence (not word of mouth or # of posts in a forum), for the purpose of building a forge lined with ceramic wood then kastolite then metrikote .. rigidizing does nothing. And i have a feeling that if you ridigized a blanket then hit it with a flame till 2000 degrees .. those fibers will start flying off anyway. 

Edited by Nicholai
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I haven't had any problems installing ceramic matting that hasn't been exposed to flame.  If wearing a respirator and safety glasses make you feel more secure then I guess it's worth the peace of mind.  If I were to reline one of my forges then I would definitely wear a respiratory and goggles.

 

Doug 

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To answer your original question:

If the rigidizer is sodium silicate (aka water glass), as I think is most often the case, then the primary means of curing are exposure to a weak acid (most often CO2) or heat it to drive the moisture out (about 250F for a couple hours).  If you apply it and let it sit for a while it will eventually grab enough CO2 from the air, while simultaneously losing the water to the air, and will thus harden.  

 

But then again, it is pretty much just another silica source (silicate!) and much more likely to lead to particles of the appropriate size to cause silicosis than ceramic wool.  The hobbyist blacksmith will never be exposed to enough refractory to worry about these things though.  The industrial workers (outside of the refractory manufacturing process) rarely get exposed to enough, usually just in large scale installation or removal projects.  

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I quite like rigidizer. It does have fairly limited benefits though.

 

There are several different reasons for using it and different people will have different priorities.

 

I don't expect rigidizer alone to immobilize fibres. I have at least a slight suspicion that rigidizing Low Body Persistence fibres will increase their body persistence. I don't see a convincing safety argument for either applying rigidizer or for not applying rigidizer.

 

I tend to view the rigidizer as something to stiffen up the insulating layer that supports the hard refractory coating used to form the hot-face.  Making the back-up layer more rigid seems likely to reduce the chances of poking holes through the hot-face layer.

 

Fumed Silica is not necessarily the same thing as rigidizer. Rigidizer is a suspension of minute Silica particles in water. The concentration of Silica that can be retained by the water is very dependent on particle size. "Fumed Silica" is a term that covers everything from about 50 nm down. As far as I can tell, the maximum concentration that can be suspended indefinitely in water (with small additions of proprietary agents to help keep the silica suspended) occurs with a particle size around 15 nm. The rigidizer manufacturers source their fumed Silica with particle sizing to suit the intended purpose. If you buy fumed Silica intended for thickening epoxy resin, for example, the particle size will be optimized for that particular purpose. 

 

I have played around with homebrewed rigidizer based on fumed Silica for thickening resins and with a "real" rigidizer from a refractory supplier. The real stuff is MUCH more effective. For the homebrew, I used Cab-O-Sil M5 and couldn't get the density above 1.015 kg/litre, even though I tried a number of different household surfactants and various things to adjust the pH. The "real" stuff had a density of 1.120 kg/l and "seemed" to be around an order of magnitude more effective.

 

I also had a play with Sodium Silicate (aka "Waterglass"). It dissolves in water and getting the solution density to the same sort of value as the "Real" rigidizer was no problem. It works remarkably well up to about 1100 degC, 2000 degF, at which temperature it melts, stops being a rigidizer, and becomes a pretty effective lubricant. The fibres on the wall of the forge where the flame impinged moved out of the way under the force of the flame. Within minutes, there was a sizable crater in the lining.

 

I would say that Waterglass is no use at all as a rigidizer in most forges. It "might" work to rigidize the blanket in a forge with a reasonably thick hot-face layer of something like Kast-O-Lite 30, but I wouldn't take the risk as the Kast-O-Lite 30 is not cheap and there's not much overall saving to be had by forgoing the proper rigidizer.  

 

I do use Waterglass rigidizer in dedicated Heat-Treat forges, where extremely tight temperature control is the objective and temperatures will not get anywhere near the melting temperature of waterglass.

 

For "normal" forges, I tend to soak the blanket in rigidizer fairly thoroughly. My reasoning is that I want the blanket behind the hot-face layer to be as rigid as I can reasonably get it, to help to reduce the likelihood of poking holes through the relatively thin hot-face layer. As Alan says, rigidizer does reduce the insulating properties of the blanket. I dug out some old textbooks, did some calculations and didn't feel it made a significant difference compared to all the other factors that go into making a forge work. YMMV. Climate and patience are factors to consider. If you live in Death Valley, liberally soaking in rigidizer is unlikely to give you too much difficulty drying things out. I live in the wet bit of England just North of Manchester and drying 2" of saturated blanket lining a 10" tube can be a fairly long-drawn-out process. I reckon on 4 weeks in what passes for summer here. I could bring it down considerably if I dried it in the house, but the domestic disharmony would be intolerable.

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