Refurbishing used furnaces...

I bought my two furnaces, a 5kg Cast Master and a 10kg knock-off of a Devil Forge, from a local artist who was clearing space in their garage for new projects. Photos are attached below for reference.

My biggest question is regarding the Kaowool lining on the 10kg furnace...there's a gap where the ends meet. Should I buy a new lining for it? That gap concerns me a tad.

I have rigidizer and Mizzou on-order from BlacksmithSupply.com, and the plan I had was to coat the Kaowool with rigidizer and then cover it with a layer of Mizzou for safety, longevity, efficiency, etc. But, as I've been looking over my furnaces today (while also working on fabricating my lifting tongs and pouring shank) I got to wondering (and worrying) that I might need to replace the Kaowool in the bigger furnace. Also, I was wondering if anyone had advice on how thick of a layer of Mizzou I should apply? I'd appreciate thoughts and insight from y'all. Thanks in advance.


This is the 10kg Devil Forge knock-off (probably originally purchased off eBay or Amazon).



This is the 5kg Cast Master.

 

The linings don't appear to have been used much and seem in pretty good condition, a gap in the lining that size will leak significant heat. Kaowool will be fragile if it's been fired so wear some sort of dust mask if you're handling it. I'd be inclined to cut a strip of kaowool and carefully poke it down with a metal rod or small wooden dowel, taking care not to compress it too much or too tightly so it still insulates well. Having the lid seal well to the wall refractory is desirable to prevent heat loss and jets of flame shooting out sideways so you'll want to build the refractory up to the height of the steel walls with mizzou or something. Others with a lot more kaowool experience will have more idea on how to get a good seal between lid and wall.

 

On the Devil Forge, the lining should be able to be lifted out if it's not too crisp. I would pull it, turn it inside out and try to stretch it a little to make up the gap.
It you are not planning on an iron duty furnace, Satanite would be the choice for coating the wool on both furnaces.
Twenty pounds will be plenty with some left over.
I find that having it in the foundry is handy. To steal the phrase from the Frank's Red Hot commercial, I put that $#!? on everything!

https://hightemptools.com/products/satanite-20-with-us-domestic-usps-priority-shipping

Here's my video on coating. Multiple thin coatings, fired to glowing red between coats.

 

I had some trouble with Mizzou om the lid of my furnace, some of it due to the nature of Mizzou (more aggregate like ready-mix concrete than smooth like portland cement) sprinkled with a bit of operator error:

https://forums.thehomefoundry.org/i...i-forgetting-sandcasting-aluminum.2162/page-2

 

The artist I purchased these from admitted they didn't use either furnace a whole lot. It was a short-term venture when they got bored during the COVID shutdown, lol. The Kaowool does feel....hard? fragile? but really only on the surface. I plan on wearing a respirator and gloves while working with it. I don't have any Kaowool on-hand, but I can buy a piece to fill the gap. Thanks for the tips!

Pulling the lining, turning it inside out, and stretching it is something I probably should have thought of already, but I didn't...so thank you for that suggestion! I'll have to try this first. It's the cheaper solution, and since I'm planning on lining over the Kaowool with refractory cement I think it should be fine.

I am planning on trying to do a small bit of iron melting, which is why I want to use the Mizzou. If I ever decide to do more than a small amount of iron I'll be inclined to look into a cupola, but for now it will be quite small amounts for small projects to make parts for the foundry (like my cope/drag alignment pins I talked about in my intro). I have ordered ten pounds of Mizzou, but maybe I didn't order enough? I have no point of reference for this, lol. I do know where I can buy a full-size bag, but I thought that would be overkill...maybe I should have just bought the big bag?

I considered the Satanite coating, but I kind-of want something that will let me melt a bit of iron now and then, and that will last longer/be more durable. The Satanite is definitely the cheaper and easier method though, and I might do that for the smaller 5kg furnace since the most it will ever see is brass or bronze.

Thanks for the video link and the Satanite link!

I'm about to head to work, but I'm going to check out the thread you linked here when I get home. I'm sure I'm going to learn a lot from it! Thank you!

Brass and bronze are what I plan to work with most commonly, but I do intend to do a small bit of cast iron now and then. I'm less concerned with speed of heating up as I am with ability to keep the furnace at-temp since my goal is to have flasks lined up for pouring and run the furnace until they're all filled. My plan currently for the majority of my casting is to make hardware (ie. belt buckles or out-of-production/historical hardware) for my leather shop.

I have not worked with dense castable before and that's why I'm scouring the forums for info as well as asking here. Thank you for the tips and advice!

 

you've already received some great info the only thing I would add

Mizzou the only place to use Mizzou is on the floor of a crucible furnace everywhere else you want to use an insulated material
KAST-O-LITE® 30 LI Plus is a Go to product for insulated refractory note the number 30 that is short for 3000F DO NOT accidentally buy 22 to save money, the furnace will eat itself , ask me how i Know !!!

that second picture makes it look like the flame will directly impact on the crucible: if it does your crucible will have a VERY short life.

V/r HT1
P.S. on a reverb or holding furnace Mizzou is a good choice because it holds up better to the abuse then an insulated refractory

 

So...I'm a bit confused. Why only use Mizzou on the floor of the furnace and not the whole thing? And why KAST-O-LITE? And I am working on sorting out how to keep the flame from hitting the crucible directly.

 

Dense castable refractories are more thermally conductive than low density fiber based insulation and have more mass to heat up. They are more resistant to fluxes and iron slag and are mechanically more durable than fiber with a coating of hotface. It becomes a trade off between durability and heating times where the dense castable is like the freight truck of furnaces versus the fiber/hotface which is the sportscar of furnaces. Once a dense castable furnace is up to temperature and actually hot, subsequent melt times get faster and faster so it's suited to industrial use where multiple melts are made and durability counts.

 

To keep the flames from hitting directly on the crucible, easiest solution is a taller plinth under the crucible, if the overall internal height of the furnace allows. Smaller propane furnaces you might just need/ only get another inch, people running diesel/oil tend to allow even more height to promote the burning of the fuel around the crucible. Below design was supposed to be either/or so I made it taller.

 

If you are going the dense cashable route, one thing you can do to give you a boost in the insulation factor is mix Styrofoam beads in the mix before you put it in the furnace. On the first fire up the beads burn out and leave voids. This is a trade off solution, it's not as strong and damage resistant as straight castable, but it's a lot more rugged than kaowool. It's not as insulating as kaowool, but it's better than solid castable. How much to mix in? You will have to experiment, the more the better but at some point it will lose strength.

 

It looks like the tuyere end of the burner on the 5kg Cast Master is set too far in and/or is not 'factory'.
Current picture of my 6kg with the Mizzou on the sides, after many firings and living outside.
Underneath the crucible is a 1+1/4" (32mm) piece of hard firebrick, not perfect but better than nothing.

 

Mark gets the gold star today.

Since you have the fiber/hotface furnaces in-hand, it makes the most sense to move ahead with the fiber repair and satanite hotface described above. You can use your mizzou to make plinths of the correct height as Tops describes above. (You won’t be wasting it). As Ironsides has shown us before, iron can be melted with propane in a fiber insulated furnace, but it’s a stretch. The small amount of time and effort required to get these small furnaces up to snuff to effectively melt aluminum and copper alloys will be well worth it. And it will give you an opportunity to get your feet wet in the melting and pouring process. If you want to build something more robust with castable refractory down the road, you’ll still have these little gems around for your lower temp one-offs, and your time will have been well spent.
The deal with the Kastolite-30 is that it offers much of the toughness and temp limits of mizzou, but has very good insulating qualities and will stand up to a waste oil burner (which your fiber furnaces will not), but this is starting to get us into a much larger subject.

Pete

 

He's also going to loose 2 inches of bore with a 1 inch lining ending up with 2 similar size furnaces.

 

That's an interesting concept, and one I'll have to look into.

The burner in the image is definitely too far into the furnace and is from transport. I'm not running the furnace like that, lol. How thick is the Mizzou that you applied in your furnace?

Thank you for the info again. Seems like the general recommendation is towards using Satanite, and though I probably won't be able to do the iron casting I wanted to do perhaps it's the smarter way to go for now.

I wasn't thinking of doing a 1-inch thick layer of Mizzou, though I have seen that recommended in other builds. One of the reasons I was asking questions in this thread was to see if anyone had a recommendation for how thick of a layer to use to protect the Kaowool without loosing to much space.

 

My guess is 1/4 to 3/8" (6-9mm), most likely thicker on floor and lower walls and thinner on top. This worked OK on the sides and bottom but did not work well hanging from the inside of the lid. I was afraid it was going to let loose and fall into the crucible during a melt.

If I were to do it over, I'd find a more suitable thinner and smoother wash coat like Satanite or Minro-Z (locally available). If I was stuck in the middle of nowhere and had a cool furnace and a bunch of Mizzou with no other options...hypothetically as I have never done this... I would experiment with sifting the Mizzou and/or smashing up the big chunks so it could be applied thinner and smoother. The trouble with the baby furnaces is that any sort of thicker hot faces will eat up the room around the crucible. First thing to go is enough room to use better lifting tongs, after that would be clearance around the crucible to promote burning and swirling of the propane.

The CAD I have been using upthread is a design for Kastolite 3000 backed by Kaowool. I should really get back on finishing that one.

 

Thank you for the reply and the input. I'm starting to lean towards the Satanite option for now. I like the durability of the Mizzou option, and the heat retention. But the case for using Satanite is a persuasive one.

 

it will be extremely hard to cast a castable refactory this thin, yes I know someone over at AA did one about 1/2 inch . Dont remember how well it held up to use

V/r HT1

 

This is a very good idea, I did this as well. No more Mizzou or another concrete for me as these are heat soaking.
My newest furnaces are made from a large paint can or a kitchen waste bin lined with two layers of 1" kaowool and the kaowool coated with Fermit. Fermit is a 1650 C / 3000 F rated Satanite like chamotte mortar available on Amazon EU sold in 3kg small buckets. I add just a bit too much water, so I can apply it with a brush. I paint the whole inside Kaowool with this and let it dry. It helps when wetting (e.g. a plant sprayer) the Kaowool first a bit. After painting I let it dry / cure for a day or two and then I put a 60 or 75W incandescent bulb into it for a few hours to make it moisture free.
The thin coating feels like 'papier maché' and it is vulnerable, so as long I don't hit it with a crucible or tongs, it lasts several heats. And when cracks appear, just repaint the cracks, let it dry and it can be heated again. My furnace can be heated to 1600 C so I can melt cast iron as well.
Even a repair after spilling some metal inside the furnace works very well. After breaking out the metal remains I fill up the holes and cavities with Kaowool leftovers, use the cement and repaint it. Hence it is a good idea to save all, even small, Kaowool leftovers in a plastic bag for repairs. When repairing, wear a face mask or respirator as inhaling small Kaowool fibers might damage lungs.
Broken or worn out crucibles can be used as a plinth when put upside down. For me this works awesome.

 

Thank you for your input! The idea of using a light for heating is nifty. We have some heat lamps for keeping chickens warm and that might work well for drying things out before an actual heat-cure. I had't thought about that before, but now I think I'm going to try that.

One of the reasons I was looking into something like Mizzou is how durable it is. And since I plan on heating/melting more than one crucible per firing I like that once it's heated up the Mizzou refractory makes successive melts faster because of the heat retention. But, I am finding out that it will take too much space out of my current furnaces and reduce my ability to fit my crucibles in there. Based on that, and the overwhelming support for Satanite, I think I'm going to use that for now.

With everything I've learned from the conversations here in this thread, as well as from my browsing around other parts of this forum and a few blacksmithing/knife-making forums, I have decided to work on plans to build a furnace from scratch that will be large enough to insulate with an inch of Kaowool and an inch of Kastolite 30, and possibly still use some Mizzou for the floor. We'll see how those plans settle out, but for now I'm focusing my time and effort (and money) on getting my current two furnaces safe and operating.