Refurbishing used furnaces...

Discussion in 'Furnaces and their construction' started by Riley Alfred, Jan 18, 2024.

  1. metallab

    metallab Silver

    I have had a furnace built from Mizzou concrete, but it soaks heat. As you say, when hot, it retains the heat well, but a well insulated furnace does not need this much as more heat is kept inside. Imagine your home walls made from just concrete. That will feel much more uncomfortable indoors as the walls soak more heat. Hence our homes have well insulated walls and windows.
    The same applies to a furnace: well insulating Kaowool keeping the heat inside.
    Even industrial furnaces designed to keep hot for a very long time are well insulated with ceramic blankets on the outside and the inner walls of newer furnaces are now made from lightweight well insulating bricks.
    Riley Alfred likes this.
  2. Riley Alfred

    Riley Alfred Copper Banner Member

    I have long imagined having a home with walls made from just concrete, lol. Given I live in a rather desert environment that makes a great insulator against the ball of fire in the sky. Most of the homes in the desert in Peru when I was there were poured concrete walls, as well as when I was in the Middle East. When you don't have air conditioning, having a thick heat-absorbing material for your walls works nicely to keep the inside of the house cool. That all said, however, I do take your point. My current little furnaces are going to get the treatment recommended here, with Satanite. We'll see how well/long it holds up. When I get to building a larger furnace from scratch in the future I will be a bit more well researched and experienced, and I am probably going to use a combination of things on that project...but for now I have two very usable furnaces that I think will do just fine once I get the rigidizer and Satanite on them.

    Also, it is interesting (though not surprising, I suppose) to hear that modern industrial furnaces are using more insulation now vs just poured refractory. Probably much cheaper construction that way, and more cost effective to run.
  3. FishbonzWV

    FishbonzWV Silver Banner Member

    I wouldn't put the rigidizer on the wool. You'll want the Satanite to absorb into the wool for adherence, the soupier the better on the first coat. That's why I recommended turning the wool inside out, just in case it was crispy.
    There is no dry out time between application and firing. Paint it on and fire it. Three coats minimum to get a good shell.
    A weed burner can be used or put your burner in. You can get it coated in a couple hours if you put a fan on it to cool down between coats. I've applied the coats with the furnace so hot it boiled the water out as I painted it on. Not recommended because it stinks and gets in your sinus and you'll smell it for hours. :-(
    I use half of an insulated fire brick for a plinth. Put three wraps of stainless wire around it and coat it with Satanite. It will crack without the wire.
  4. metallab

    metallab Silver

    That is exactly the reason why I use the bottom half of a worn out crucible upside down as a plinth. They won't crack. And I paint it to make it a rougher surface and every time I put a crucible on it, I put a piece of cardboard under it. The ash and carbon remains will prevent sticking it to the plinth.
    Riley Alfred likes this.
  5. Riley Alfred

    Riley Alfred Copper Banner Member

    I love the idea of the piece of cardboard. Carbon is sometimes used to keep sand casting molds from sticking as I've been reading, and it makes sense that it would keep your crucible from sticking to the plinth. I'll have to give that a go!

    We've had very, very humid weather lately (inches of rain for days) so it hasn't been good weather for doing anything with the furnaces. But once things dry out a bit I'm going to pull them both out and dress them up. I'm probably going to order the Satanite this week so by the time it gets here I should have some dry ground to work with (everything flooded here so saying it is muddy is an understatement).
  6. Riley Alfred

    Riley Alfred Copper Banner Member

    So...I'm about to order my Satanite and a bit of Kaowool/Inswool to do the thing with my furnaces. I was planning on ordering from but their website looks like it hasn't been updated in six months so I was a bit concerned. Has anyone ordered from them recently? Are they a good vendor to purchase from?

  7. FishbonzWV

    FishbonzWV Silver Banner Member

    I've ordered from them four times. Never disappointed.
    They have upgraded the website because my old bookmark to the Satanite didn't work.
    Riley Alfred likes this.
  8. Riley Alfred

    Riley Alfred Copper Banner Member

    OK, cool. Thank you for that. I'll be ordering the Satanite from them now then.
    FishbonzWV likes this.
  9. Riley Alfred

    Riley Alfred Copper Banner Member

    My order from HighTempTools arrived today! I have things to deal with at the local Veteran's Hospital tomorrow that will have me busy until the afternoon, but I'm hoping to get started on the refurbishing of the two furnaces as soon as I get home.

    Since I have some Mizzou (10lbs of it) I'm think I'm going to use it to re-line the bottom of the furnaces as someone here had suggested previously. The reason I think I want to do that is I feel it will hold up better if I spill something or have a crucible break, etc.

    With the bottom taken care of, my next planned step is to make sure the Kaowool wraps completely around the inside of the furnace on the big one and then use rigidizer on the insulation for both furnaces.

    Once that is done I plan to use the Satanite that I got from HighTempTools to line the hot-face of the furnaces. I'll probably do at least a couple coats. I got 20lbs of Satanite because I'm sure I'll need to do repairs along the way. Not sure how much I'll need to get started though.

    My next planned step is to coat the cured Satanite with a layer of ITC-100.

    With all that done, I should have two nice little furnaces ready to melt on the regular!

    I am also hoping to get back at the lifting tong/pouring shank project on Saturday.
  10. FishbonzWV

    FishbonzWV Silver Banner Member

    Let me re-iterate.
    Three coats minimum, four or five on the floor.
  11. FishbonzWV

    FishbonzWV Silver Banner Member

    I agree.

  12. mytwhyt

    mytwhyt Silver

    I has been my understanding that the fiber blanket turns black before it turns white is because it needs to burn out the organic binders first. I'm interested in how it behaves when you turn it inside out without them. I've never done that so am curious.
  13. Riley Alfred

    Riley Alfred Copper Banner Member

    I hear you. And I understand what you are saying. I am concerned, however, by all the information I am hearing and reading from multiple sources (for example the blacksmithing forums, because I am also looking into acquiring a forge within the next year or so) saying that the only thing that fully stops the fibers of the ceramic insulation blanket from becoming airborne is using rigidizer. Because of how carcinogenic and dangerous breathing those fibers is, the recommendation I have seen over and over is to always use rigidizer first. Even if you're going to paint the Kaowool with multiple layers of Satanite or encase it in some other refractory.

    Also, to be clear about my comment, "I'll probably do at least a couple coats", my plan is and has been (since being persuaded to try the Satanite) to do three coats and see where things stand.

    I liked the suggestion of using the Mizzou on the floor of the furnace for the reasons I stated above. I understand that the more mass the furnace has the longer it takes to heat up. I have, from multiple places on this forum and elsewhere, learned what the benefits of a low-mass furnace are. I also know that I like to do something once and not have to do it again. I don't like having to repair something over and over when I could have just done things from the start in a way that avoids (what I consider to be) unnecessary maintenance. With this furnace refurbishment project I am hoping to utilize the advice given by everyone here to the best of my ability given that all y'all have far more melting and casting experience than I do. There seems to be overwhelming suggestions and preference for using Satanite on top of the Kaowool, hence my choice to move in that direction. But there is also recommendations for using the Mizzou on the floor of the furnace, both here and in the blacksmithing forums, and I like the reasoning given in both places. Hence my decision to do that. Seeing as I will not need 10lbs of Mizzou to do the bottoms of both furnaces I will be making some plinths out of it as well. That was also a really good suggestion.

    I had never heard this before. That is interesting. The fiber blanket technically has no "inside" or "outside" save that in my case because the furnaces have been fired several times already there are scorch marks on the Kaowool on the "inside" that might cause issues with trying to adhere any sort of refractory or other substance. Because of this, I am going to take the ceramic fiber blanket out of both furnaces and flip it around so there is a clean "face" on the inside of the furnaces to work with as I re-line them.
  14. FishbonzWV

    FishbonzWV Silver Banner Member

    That's fine, do what you feel you must do.
    My understanding is that rigidizer is used to seal the wool if no other coating is used.
    Since you have both products on hand, I was wondering if you could do a comparison test on a couple coupons of wool. One with and one without, then peel the Satanite off. I would definitely like to know if there is a difference in the penetration rate of the Satanite into the wool.
    The floor and wall becomes a solid shell so they are stable. The lid is where you want maximum penetration because gravity never stops.
  15. mytwhyt

    mytwhyt Silver

    By the outside i meant the other side in your furnace.. I believe the thinned Satinite would better penetrate the blanket without the rigidizer and seal the fibers as well. I get by with a very small propane furnace. I only do small castings.. Small aluminum propane furnace.. | The Home Foundry.
  16. Riley Alfred

    Riley Alfred Copper Banner Member

    I should have enough scrap Kaowool to do a test like that.

    The lid is the one thing that has me mildly concerned... When I bought the furnaces the larger one had a piece of steel wire wrapped around the lid holding the Kaowool in place, which I do not like at all. I haven't settled on a solution yet and I'm open to suggestions about how to make the insulation stay in the lid.

    I'm trying to understand what you're talking about, but I must say I am still a bit confused. I'm sure the thinned Satanite penetrates the wool well. I'm convinced by what I've been learning that the Satanite will adhere just fine. And, as FishbonzWV has asked, I am going to do a test with some scraps of Kaowool to see what happens.
  17. mytwhyt

    mytwhyt Silver

    I thought we were talking about the fiber blanket in your furnace that you're planning on reversing to use the other side. And that satinite would serve the same purpose as ridigizer to control any loose fiber as well as boost its temp range. Sorry for the confusing way i said it. Will be interested to see how the test pieces come out.
    Riley Alfred likes this.
  18. Riley Alfred

    Riley Alfred Copper Banner Member

    We were/are talking about the fiber blanket in the furnaces I have, and I am planning on removing it and using the un-charred side on the inside of the furnace as I (and others here) feel that the Satanite will adhere to it better than on the charred side. As the Kaowool insulation doesn't look overly charred/damaged it was suggested that I try this vs spending the money to replace it.

    I know that there is thought that Satanite will serve the same purpose as rigidizer to control loose fibers in the Kaowool. I'm not sure I'm willing to subscribe to that yet, but hence the test I'll run soon. We'll see if the Satanite adheres to the rigidized Kaowool as well as it does the non-rigidized. I'll probably do this in my small furnace as I've already disassembled the large one. I have a busy weekend ahead despite having tomorrow off work, but hopefully there will be time for some fun with the furnaces.

    And don't feel bad for me being confused. I'm autistic and often misunderstand what people are saying, be that in-person or via text. It's likely that the "culprit" for my confusion is my own brain.
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I've been following along and have worked with all the materials being discussed.

    I honestly don't see any need to reverse the wool. In fact, the side exposed to flame and heat often becomes more brital and breaks down a bit when reformed. I doubt there will be much difference in how the Satanite sticks. As has already been said, expecting the Satanite to remain suspended from the roof merely by the strength of the wool will eventually become a losing fight. Better if it has a little internal dome to it or some mechanical support.

    As far as the health concerns with ceramic fiber, these often get overblown. It's true it is friable and can become airborn, and also true that it is a carcinagen, but like most carcinagens, it's related to exposure and most of the time were talking about prolonged and daily exposures with these risks. In an outdoor furnace, the chance that you would be exposed to any measurable amount is remote. In fact your exposure to silicates is probably much higher at the beach on a breezy day and probably much, much, greater when you open and dump your shop vac and find yourself standing in a cloud of all those unknown fines that have become airborne......but you don't see peple preeching these hazards. Ceramic fiber is a great insulator. Ceramic fiber is not a good hot face and will break down in the corrosive, reducing, combustion gas, furnace environment. Once the ceramic fiber is coated with Satanite, both issues are adequately addressed.

    Rigidizer is coloidial silica (CS). It is watery thin. If you try to apply it to ceramic fiber with a brush, that location will absorb it like a sponge. You'll then need to dry it with furnace heat because otherwise it will just sit there soaking wet like a wet wool sweater. It's expensive too. Even your size furnace could easily make a gallon dissappear. You'd be better off applying it with a spray bottle. The surface is the place where friable fibers come from. And yes, CS is also an aluminosilicate so the same safety discussion applies. Applying rigidizer and Satanite is belt and don't need both but rigidizer won't do much for hot face durability so why bother?

    -My 2 cents

    Riley Alfred likes this.
  20. Riley Alfred

    Riley Alfred Copper Banner Member

    Your perspective is appreciated! FWIW, I often over-analyze, over-think, etc. most projects I work on. And then I over-engineer them. Sometimes that pans out well, other times it works to my detriment.

    The roof/lid of my furnaces is what has had me puzzling the most tbh. I had thought about drilling a couple of holes in the side of the lid and running a couple of cross-bars of stainless steel with threaded ends (to secure with nuts) so there would be a physical mechanical support in (roughly) the middle of the 2-inch thick lid insulation. I've seen some folk's furnaces with a more domed internal shape and I like the look/concept of this. I'm guessing it is possible to cut the Kaowool easily enough to shape it this way and then paint the Satanite on it? The dome shape should provide the structure needed to maintain its position and shape, right?

    Fair points, all of this. Especially once my brain mulls it over a bit. Your point about exposure at the beach or dumping a shop vac is one I hadn't thought about but makes absolute sense, and your'e right that no one really "preaches" about the dangers of breathing in the dust from your shop vac which is going to be concentrated vs just the general shop environment.

    As far as safety when handling the insulation, rigidizer, refractory cement & mortar, I am following all the recommended precautions. Long sleeves, gloves, respirator, etc. My time as (among other things) a safety NCO in the Air Force has me a bit....overfocused on safety, at times (dealing with working in asbestos contaminated buildings didn't help, lol). My brain focuses on a thing and then it's a struggle to get it to change gears.

    The analogy of a "belt vs suspenders" is neat, and one that I easily grasp. As I've been reading, watching videos, and talking with folks that use similar furnaces (and forges), I have been constantly told about the dangers of ceramic wool insulation and had so many people insisting the "only" way that you stop the fibers from becoming airborne is with rigidizer. Through this thread here I'm seeing more and more folks (with far more experience than I have, let's be honest) siding in favor of just using the Satanite and forgoing the rigidizer. On its own that doesn't always tell me much, but your breakdown and all that has really helped me understand the point better.

    The cost of the rigidizer wasn't too bad (I did end up buying some when I ordered everything else). I ordered two quarts of it, expecting that would be enough for the bigger furnace. If you're saying my furnace could soak up a gallon of the stuff though, that's far more than I was expecting and despite having it on-hand, I certainly don't have enough. Add on your "belt vs suspenders" point, and I'm thinking perhaps I don't do the rigidizer after all (though, I still want to do a test with the Kaowool, rigidizer, and Satanite like was mentioned in previous posts). A spray bottle was how I had planned on applying the rigidizer, fwiw, not that I suppose it matters at this point.

    I appreciate everyone who has taken the time to post on here and offered your experiences and opinions. There's nothing compelling anyone to weigh in, so I wanted to make sure to convey my thanks for your time and consideration.

    I was originally planning on heat curing the Mizzou floor on the bigger furnace today (short video of me working on that to come). I was also going to get to welding on the lifting tongs & pouring shank...but alas, it is raining today. I can't weld outdoors in the rain, and I don't want to tend the furnace in the rain either. So today will be a relax inside day instead. Maybe I'll figure out what I'm doing wrong with exporting my edited videos.

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