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Author: Subject: Refractory with flour or saw dust incorporated?
RogueRose
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[*] posted on 3-11-2016 at 12:01
Refractory with flour or saw dust incorporated?


I need to make refractory for a forge (and later a kiln) and have used clay, sand, cement and perlite in the past and it works alright but degrades at high heat and is pretty conductive of heat even with 3 inches or so.

I was reading the thread about refractory made from bread but I don't think that is suitable in this instance. I am thinking that some aspects of that process may be integrated into the mixture like adding flour to the mix and when it is heated to forging temps (1500-2200F) the flour would carbonize (though IDK if the gases released would crack the material). I've heard some fire bricks are made with saw dust and it is burnt out as well - so that may be an option as well.

With the bread/flour refractory post it mentions yeast but I'm wondering if baking soda or ammonium bicarb could be used to generate the gas bubbles and if that would translate to mixing into the cement/clay/sand refractory.

So I guess the main question is what is the major difference between heating saw dust to a high temp and flour. Does it give a different carbon structure and which would be more beneficial to refractory material.

**NOTE - IDK if this should be in the referenced thread but it seemed like it was somewhat tangental, obviously feel free to move/merge if appropriate.
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mikeehlert
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[*] posted on 3-11-2016 at 12:07


Quote: Originally posted by RogueRose  
I need to make refractory for a forge (and later a kiln) and have used clay, sand, cement and perlite in the past and it works alright but degrades at high heat and is pretty conductive of heat even with 3 inches or so.

So I guess the main question is what is the major difference between heating saw dust to a high temp and flour. Does it give a different carbon structure and which would be more beneficial to refractory material.


Sawdust will be coarser and less pure due to pitch and etc. This is a old time trick to reduce density and/or increase insulation rating of brick. Search the used book story for some old texts on refractories if you want formulas.
Your objective here is to make it as porous as possible while having enough strength for the application. You will need to dry and burn it our VERY slowly - days.:)




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aga
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[*] posted on 3-11-2016 at 12:35


Quote: Originally posted by RogueRose  
I need to make refractory for a forge (and later a kiln) and have used clay, sand, cement and perlite in the past and it works alright but degrades at high heat and is pretty conductive of heat even with 3 inches or so.

Perlite, sand and clay should have been OK.

Got a photo of how it turned out ?

It'd be interesting to see what actually happened to those materials.




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Texium
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3-11-2016 at 13:01
RogueRose
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[*] posted on 3-11-2016 at 15:07


Quote: Originally posted by aga  
Quote: Originally posted by RogueRose  
I need to make refractory for a forge (and later a kiln) and have used clay, sand, cement and perlite in the past and it works alright but degrades at high heat and is pretty conductive of heat even with 3 inches or so.

Perlite, sand and clay should have been OK.

Got a photo of how it turned out ?

It'd be interesting to see what actually happened to those materials.


I'll see if I can dig up the original photos when it was first "poured" (it was actually packed as it was a sticky thick mud/clay texture). The mixture was pressed around the circumference of the bucket, to thickness 9towards the center) and added layers while working around in a circle gradually getting to the top - like a long snake.

I think the recipe was something like:
3 parts clay
3 parts sand
2 parts portland cement
3 parts perlite

add water until a cookie dough like texture.

Add water to the clay to saturate then mix in the sand/cement/perlite mixture about 10% at a time. I ended up pouring a lot more perlite in and mixing at the end and it seemed to mix in pretty well.

This is after about a year of moderate to heavy use and it has slowly eroded away from 2.5" to about 1.25".
refractory.png - 266kB

The reason I was looking at the flour or saw dust was that the metal bucket would get red hot when high heat was used. I was thinking it might give better or additional insulation
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careysub
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[*] posted on 3-11-2016 at 15:32


If you want something with significant insulating value try using perlite or coarse vermiculite with Hercules furnace cement (rated to 1650 C), or sodium silicate/ground vermiculite cement to bind it together.

Perlite is "pre-bubbled" and its service temperature is about 900 C. Vermiculite is good to 1200 C.

I would not try experimenting with using any combustible in something you are building for use. Experimenting with samples just for the heck of it is another matter entirely.

[Edited on 3-11-2016 by careysub]




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ziqquratu
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[*] posted on 3-11-2016 at 15:51


Sawdust can potentially work, but you might try shredded polystyrene foam, or foam beads - the idea is that it should burn away cleanly upon heating, leaving plenty of insulating voids in the refractory, and without leaving behind various fluxes which might impair the refractoriness of the material. You're also not limited to the working temperature of the additive, as you are with perlite or vermiculite (after a careful initial heating to burn it all out, of course!).

The limit, of course - which will be true for any method you use to introduce voids in your material - is that with more voids, the material will be a better insulator, but it will be less mechanically resilient. You have to work with that trade-off to find a suitable balance (or you have to protect the outside of your fragile insulating material with a layer of denser, less insulating refractory).

Check out Alloy Avenue - they're all about building furnaces for metal melting, and there are many mixtures for home-brew refractories discussed there.
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RogueRose
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[*] posted on 3-11-2016 at 17:46


Quote: Originally posted by ziqquratu  
Sawdust can potentially work, but you might try shredded polystyrene foam, or foam beads - the idea is that it should burn away cleanly upon heating, leaving plenty of insulating voids in the refractory, and without leaving behind various fluxes which might impair the refractoriness of the material. You're also not limited to the working temperature of the additive, as you are with perlite or vermiculite (after a careful initial heating to burn it all out, of course!).

The limit, of course - which will be true for any method you use to introduce voids in your material - is that with more voids, the material will be a better insulator, but it will be less mechanically resilient. You have to work with that trade-off to find a suitable balance (or you have to protect the outside of your fragile insulating material with a layer of denser, less insulating refractory).

Check out Alloy Avenue - they're all about building furnaces for metal melting, and there are many mixtures for home-brew refractories discussed there.


Thanks for the info and links!

I also have some MgO which is supposed to be used in very high temp refractory although I'm not sure what form it needs to be in - mine is flake and agriculture grade.

I have a lot of CaCO3 which could be interesting as CaO is used in some cements and calcination of CaCO3 gives up the CO2. The weight of a mole of CaO is about 50% that of CaCO3, so there would be some loss of mass when heated to 1517F. I think this may give some insulation quality as it would create small voids where the CO2 has escaped.

I'm also wondering about adding some kind of matting or fiberous material something like fiberglass, ceramic wool/fibers or some aesbestos like material that is thread-like. I'm thinking that 2-3" lengths of these fibers/threads could be mixed into the mixture to give a binding element allowing it to all securely mesh together better than without something such as this.

Another idea I had was steel, copper or some high temp metal/metal alloy thin wire mixed in as well. Copper may not be the best choice but it's ability to conduct heat may be beneficial to keep the part closer to the flame from melting but it would also wick heat away from the center along the wire.



[Edited on 4-11-2016 by RogueRose]
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macckone
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[*] posted on 3-11-2016 at 18:16


Polystyrene is best, no fluxes. And you can get it for practically free.
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wg48
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[*] posted on 4-11-2016 at 02:06


Saw dust seems perfect for producing a low density insulation after the insulation is fired. The grain size is similar to the voids in commercial low density firebricks compared to the much larger polystyrene balls. The size of the voids in the final insulation may be important for good insulation performance.

Obviously both are going to give off horrible fumes for the first firing and less so for the subsequent firings. Perhaps the saw dust could be converted to charcoal first which would eliminate the problem though carbon monoxide would still be produced.

Depending on what temperature your trying to get to and the components of your insulation cement or CaO should not be used except perhaps in a few percent to act as a flux to sinter the more refractory components of the insulation like MgO, SiO2 and Al2O3 as explained in http://alloyavenue.com/vb/forum.php

Free uncombined CaO absorbs moisture and CO2 expanding as it does so. When heated to about 1000C it converts back to CaO which destroys its integrity. Meaning the insulation crumbles.

MgO is a +1500C refractory (and compatible with metal electric elements) but that also means its sintering temperature is very high. You can heat pure MgO as high as you can with a propane air torch and it will still crumble after it cools. So to get it to sinter at a low temperature you need add a flux.

The Alloyavenue explains all the details in this thread http://www.alloyavenue.com/vb/showthread.php?5-DIY-Refractor...


[Edited on 4-11-2016 by wg48]
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macckone
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[*] posted on 4-11-2016 at 17:20


You have to put the styrofoam in a blender to get the right pore size. Or use micro beads.
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