Perfect mix for a core

I'm getting ready to pour my core and I wanted to check on the perfect mix ingredients. From Matt Walker's video, he used a 50lb bag of Lincoln 60 fire clay, around 2cf of perlite (a 1:1 ratio), a handful of shredded fiberglass and a gallon of furnace cement. The amounts will vary I assume based on the size of the core and the desired thickness, but is this the going recipe for core casting?

I called up a ceramics place and they recommended using Kast-o-lite Insulating Castable Refractory (detailed information below). There other suggestion was XL+ Refractory cement. Both are pretty reasonably priced, (around $25/gallon) For this ingredient, is more always better? If I was feeling flush and wanted a super core, would I add two gallons or does 1 gallon do it (whatever it is and more is wasting it?

The last piece is the fiberglass. Did Matt just rip or cut up some left over batt insulation and add it to the mix or is this an ingredient that you buy somewhere? I've got a clay mold place that sells bulk fiberglass by the bag. I can do either. And roughly how much should one add? Is more better?

I don't think I'm up for making my own water glass yet (another ingredient I've seen added to core/riser mixes), but anything else worth considering? Is there any kind of consensus in the community that this makes a great core or is this mix more still under investigation?

Thanks for any thoughts - Rick

ps - I know I shouldn't have made the burnable form out of OSB, but I didn't have any redwood/solid wood around and I'll make sure to burn it off on a windy day out in the open


Kast-O-lite 30 LI Insulating Castable Refractory:

This is a 3000 degree F, High Alumina Insulating Castable Refractory for making forges. You can cast the entire refractory liner for a forge or just the floor using this material!

- 3000 degree temperature rating
- Highly flux resistant
- Low thermal conductivity = highly insulating compared to Mizzou
- Density = 90 lb/ft^3

- This material has a thermal conductivity of 4.54 btu-in/hr-F-ft^2 at 2000 degrees F and conducts almost 40% less heat through the forge shell at 2000 degrees F!

Rick,
Have you had a chance to make the casting and burn out the form? It will be very interesting to see how this cast material handles the temperatures. I had also commented on your jacuzzi thread. Based on the pictures the jacuzzi J-tube appears to be on a slab rather than the form in this thread. Any plans to bring the two together?
AL

Thanks for checking in. I did build the core, and I started a different thread to follow up as I got into the questions about using brick channels sandwiched between permaboard as my ducting and having the whole thing under a aquaponics water table. The core part of it has been great. A few significant cracks, a bit of crumbling around the bottom of the feed tube, but mostly great. It burns hot, lights easily (thanks to my fan and aside from wishing I had installed some sort of easy ash removal system, I love it. I've still got it in the wood box I poured it in. Even after burning it hot for six hours straight, the wood on the outside is barely warm. I haven't tried melting copper yet, but aluminum cans disappear quickly and I've hit 800 degrees on the top of my barrel with a 5" gap from the top of the riser. As far as I can tell, it's working great.

The one I showed in the jacuzzi picture is in a different part of the yard, it's a fire brick core on a piece of permaboard on 3" of sand with a 6" steel riser, 12" duct around it filled with 10:1 perlite to clay insulation and a barrel to help direct the heat into the radiator.

Thanks again for checking in. I've definitely caught the bug about experimenting with rocket mass heaters. I've been able to keep the water in my aquaponics system around 70 degrees and keep my greenhouse in the low 60's (I'm in northern california, so it's not that impressive, but it gets down to the low 40's on occasion . I'm still messing with how to make the jacuzzi system possible and thinking about a next project of a rocket stove/pizza oven combo

Hey Rick; When I poured my cast core following matts video, I found that I needed 2 bags of lincoln 60 not one as he recomended. I suspect that he cut up batts to get his fiberglass, but I went with the pre bagged stuff for concrete. I used about 3/4 of a gallon of refractory in my mix and I wished that I had used more. If I was pouring a new one I would buy extra refractory mix and it to use at the feed tube end and burn tunnel roof! You will have wear issues there that can be easily patched but a stronger mix of refractory on that end should help. As far as waterglass goes you already have it inside your refractory mix ! sodium silicate (waterglass) is part of a refractory mix. After I had my core built , I had wear issues at the feed tube. I bought (on ebay) a 40% solution of sodium silicate and (painted) the inside & bottom of the feed tube. Although ultimately I replaced my cast core with a firebrick one, the cast core was way superior to the firebrick one in that it heated up better and faster than the firebrick does. My current rmh is in the greenhouse/studio. The next will be in my auto shop and that will be firebrick core and matt walker 1/2 barrels in the mass. After that I will build another cast core (walker style) that will go in my home, where i can sit next to it and feed wood carefully . In the studio/greenhouse wood was getting stuffed into the feed tube ... busted the sides to where at the end of that winter my 7.5x7.5 feed tube hole was literally the same size as the five gallon pail that I had used as a feed tube extension. Good luck with your builds .. now you are officially a rocket scientist !

Thomas - thanks for the comments. I probably ended up using closer to two bags of fire clay as well, and most of the 4 cf bg of perlite. I ended up not using any fiberglass, and just a 1/2 gallon tub of furnace cement.

I get your point about wishing to have used more furnace cement, and I've seen places where my core is crumbling and brittle. The trick I did that I'm really happy with how it's working out is using pieces of split fire brick to form the feed tube. I had seen both Matt's video and your posts and I didn't want to have to patch that area again and again, so it seemed like a hybrid idea could work really well. Matt wrote back afterwards saying it doesn't work because the core dries and pulls away from the fire bricks, but my core is as dry as it's gonna get and there are just tiny gaps around the bricks. I had way bigger problems with drying cracks in general than I did around the fire bricks. The fire bricks form a perfect feed tube, I can jam stuff in, break branches as I put them in and not worry at all about damaging the feed tube. You can see some tiny gaps in the picture at the bottom, but nothing that seems like a problem.

The other piece I saw somewhere (I can't remember the thread), the person made a metal insert for their feedtube that both protected the feedtube from wear and tear, and, because most of the metal was outside and exposed, it served to keep the feedtube cooler which helped cut down on wood gassifying at the top of the feed tube rather than down inside the burn tunnel. I'm not a welder and couldn't easily see how to incorporate that into my design, so for now, I'm happy with my fire brick lined feed tube.

I read about your future plans for the Walker style poured core for your studio, so here's my thought for my next project. What about a hybrid core? The problem with poured cores is they're too brittle and require a lot of maintenance. The problem with a fire brick core is they take a while to heat up and even when they do, they dissipate significant heat from the fire. What about a hybrid core that gives the best of both? I'm thinking to follow the rough plans for a poured core, making an outer box and an inner form. I'm thinking to use 10-1 perlite/clay for the covering, mainly because I want serious insulation. The essence of the core is made of split fire bricks. In the bottom of the box I'll put 3" of the 10-1 perlite clay to form the insulative bottom of the core. Then I'll lay six split fire bricks sideways, side to side (6 x 4.5" = 27" length). I'll set the core form on top of the fire bricks, and place fire bricks around the form. There will need to be some brick cutting depending on the size core you want, but cutting bricks with an angle grinder has been pretty easy and cutting split bricks isn't bad at all. The sides of the core, top, feed tube, burn tube and riser are all formed out of the split fire bricks. They rest against the wood form to hold them in place and the 10-1 perlite/clay mix is packed around. If you want to go for perfection, I'd use a fire caulk to help seat the bricks so there are no gaps and the core is air tight and solid.

So now you have two great attributes for your core. First, it's as strong as fire brick, so no worries about abrasion, wear and tear or needing to patch it after a few weeks use. Second, the split bricks will take some time to heat up, but now, the majority of the core is much more insulative than Matt's core, and my guess is it will heat quickly and once it gets hot, it will do a much better job at redirecting all the heat from the fire back into the fire, better than a fire brick core or Matt's poured core. 1-1 perlite/clay is too much mass and too much possibility for heat transfer. Another piece I wondered about, that it might actually be good to have a certain amount of mass that gets hot in the core. When I feed my fire, I often don't get back to it quickly enough to put in new wood before the last bit of wood has gone to embers or is just about there. Restarting the fire again and again is a pain. But, when the core is hot, most of the time, I just add a few bits of kindling and in a minute or so, it relights and is good to go again. The split firebrick would retain enough heat to help with relights and finishing burns yet be significantly less mass than a full fire brick core. The extra insulation around the fire brick will help it heat faster and once it gets hot, redirect much more of its heat back into the fire.

The bummer for me is, I don't have any place where I need another heater I live in Northern California and it's been 70 degrees during the day for most of February. It gets down to the 40's at night, so I do heat my greenhouse, and we do heat our house, but my wife isn't a big fan of this project, so I have a design in search of a need

Thanks for all the casting mix information and pictures. I find the pictures the most motivating. I think you have some potential with the cast and brick hybrid. Good way to increase the wearability in the feed tube. I'll have to go find that other thread now.