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MonkeyForge's refractory experiment

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Hi everyone, I have not had a lot of time for hot work and forge experimentation until recently so, bear with me while I get rolling again.
since I am occasionally quoted and referred to I thought it was time to share my findings so far. Also full disclosure: Like Mikey I was, at least at the beginning inspired and guided by he Mr Hansen's work on digital fire. Also, do not see this as a scientific truth, I have done some stuff but not nearly took enough measurements to conclude anything.

- I am still messing around with this
- Even if you have a fool proof formula this may not be economical
- It "works", forge gets hot but the process of making refractory needs work.
- Insights welcome

initially I used this mixture to shape the flame facing side of the forge. This may have bitten me in the rear end, more on that later.

- Zircon             66.91%     (or 82.19% minus water and paper)
- Bentone         2.23%       (or 2.74% minus water and paper)
- Molochite     12.27%     (or 15.07% minus water and paper)
- Paper fiber     3.72%
- Water             14.87%

Unless you have a sure fire way of homogeneously distributing the paper fiber and provided the fiber size is consistent, I would forego adding it. Again, more on that later.

The parts:




Also see This post

As a refractory coating this same recipe works well, provided you do not use too much water and give it ample drying time.

The recipe for the dry mix would then be

- Zircon  82.19%
- Bentone 2.74%
- Molochite 15.07%

For a clay-like consistency (still very sticky):

- Water             14.87%

For a paintable mix to use as coating:

Add water to dry mix until specific gravity of 1.25

I contrast to the following I would not omit the molochite. for use as a coating see this post


In any case I had big cracks forming after some time, this can be due to a number of reasons, none of which I have confirmed.

The following pictures are before and after the patching, then after firing the patched flame face.

I made this thing and after approximately 60 hours of heat this happened:

My best guess here is as follows:

- Different thermal expansion properties between my home-made parts and the cement holding the parts together caused stress on the whole
- Paper fiber not equally distributed caused extra weaknesses (fault lines)
- Relatively cold ambient temperatures increased the impact of above factors


I made some more "coating" in this case just adding water by the teaspoon until I had a paste and patched the largest cracks,




I fired the forge again:


After patching and firing

after patchnig baked

The next pass will be more of a coating pass to get rid if the irregularities, so same mixture, more water. This will even out the flame facing surfaces.

So yeah forge still gets hot, I still need to mix some coating to fill in the little cracks, will keep you posted. Hope this is useful to someone, any input i welcome.






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6 minutes ago, Goods said:

Any thoughts on adding evacuated alumina spheres? Would add some insulating value.

This was the original idea with the paper fibre. Gives it dry strength before firing,,( it did so all good there), fibres burn out in the firing process leaving cavities, in theory increasing insulation. So my thoughts would be similar regarding the spheres, provided you can achieve an equal distribution throughout the clay body. 

For now it is still fun.



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Thanks Frosty, just read back my post. Outside if editing window, of course. For mixing the coating it is better to add dry mixed ingredients to moving water. I got that backwards. For creating the clay body i like to do the reverse, just because it makes managing water content a bit easier (even if more messy/ sticky)

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The new forum update limited the edit time window to something ridicules like 3-5 min. I have found if there are no posts after yours starting a new post will merge the new & old together. Good to see the MonkeyForge back online.:)

I can't control the wind, all I can do is adjust my sail’s.
Semper Paratus



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The short editing window takes getting used to. To Add to Randy's trick above, when posts are merged the edit window is reset so you can edit if you're quick. . . Sometimes. If too much time passes posts aren't merged but that window is longer.

Frosty The Lucky.

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Good to see you back, Marrten. Thank you for the update.

I seem to remember that Mr. Hansen mentioned considerable shrinkage in his own parts, during firing. So, it does not seem likely that any tweaking of ingredients will allow a rigid structure of more than single parts. Perhaps, the best path would be to fire each part, and then trap the whole assembly within some outer layers of rigidized ceramic wool. Possibly to even surround the rigidized wool within a structure of Perlite, which is glued together into a monolithic shape, with water glass? Maybe encased within sheet-metal? This sounds more complicated than it is, because the wool blanket encased part, could simply be slid into a metal box, and the Perlite poured in; then the water-glass poured over the Perlite, and the whole assembly allowed to harden. A small hole in the rear face of the box, which is temporarily at its bottom, could allow any excess water-glass to pour out, into a container for reuse.  

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Thanks Mikey. Randy thanks for the tip.

I accounted for the shrinkage part, what you describe is more or less what I did. I built the floor and the roof, had the fired in a kiln so all the firing shrinkage was accounted for. I then took an old pressure vessel, cut it in half lengthwise and put a hinge in. Built the forge around the ceramic parts and closed the lid.

My mistake here I think, again not confirmed, is cementing the floor and roof together as well as cementing some parts made out of kiln-shelf to the rest. I could at least have left some gaps/seams to account for the parts expanding and contracting during heating cycles. (in a lot of ceramics kilns the bricks are held in a frame rather then cemented together for exactly this reason)

Drying shrinkage, firing shrinkage and thermal expansion/contraction are 3 different things that should be accounted for at different stages. The coating recipe does not seem to suffer from excessive shrinking, at least not when applied in thin layers and as long as water content is not too much higher or lower than mentioned.


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On top of the swelling and than contraction of the bentonite, which is simply an essential ingredient in this formula; expansion/contraction during thermal cycling was always a major problem in hard refractories, until Kast-O-lite 30 was introduced into the market. I remember what extremes old timers went to, in avoiding major cracking in their casting furnaces back in 2000, when I started hunting for "the right refractory" to use a the flame face layer in my own equipment.

  The difference seems to be mainly due to the addition of bubble spheres. The voids they leave behind, are suppose to interrupt cracks, before they can get very far. The resulting mini-cracks are suppose to provide stress relief; anyway, that's the theory:rolleyes:

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1 hour ago, Mikey98118 said:

back in 2000

Back in the 2000's, almost feel old :)

Like I said to David, this should be feasible and it was my original intent with paper fiber.

Speculation below:

I want to try it but keep in mind that these spheres are not created equally, some are mostly silica, some are alumina/zircon +alumina. All of which have different melting points, do we want glass in in our high zircon refractory or just cavities, or both?

That being said:

- As a monolithic refractory, Still testing and tweaking.

- As a refractory coating as listed in initial post: pretty confident.

If only we had the magic recipe that did it all :)


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26 minutes ago, MonkeyForge said:

do we want glass in in our high zircon refractory or just cavities, or both?

Well, I believe the glass spheres simply melt into the refractory mix, during firing. However, the amount of glass they introduce is insignificant; it should make no practical difference in use ratings. The point of alumina and zircon spheres, is that they add to the strength of cement mixtures. My thought is that, in a refractory "so what?" We aren't designing bridges:)

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We have all failed to address the central point of your experiment; to produce a flame face material that is a hard, and re-emission layer; rather than a mere surface. I note that the burner in your forge does not look to be any advanced design, yet the exhaust opening appears as yellowish-white. I would surmise that it is above 2700 F. May we assume that it is also mechanically tough?

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I took Maarten's refractories in the kiln wash direction rather than as a hard inner layer refractory. What I have available for a reasonable price is bentonite drill mud and we did a little experimenting. The 97% zirconia - 3% bentonite works well as a kiln wash. 

A factoid about bentonite that doesn't get talked about much is it's use for plugging cupolas. It's placed on a round board on the end of a rod and shoved into the tap wet. The bentonite dries and fires hard directly from mud to hard in contact with molten slag and iron.

Sooooo, I took a heaping tbsp. dab of 97/3 mud and dropped it on the floor of my forge at mid yellow heat. It slumped like a cookie but then it foamed up and fired like ceramic in less than a minute. It's not that it's useful and I didn't feel like tinkering till it was but it was kind of cool. Get the right ratio and have a high zirconia foam refractor would be kind of neat. 

I killed a garage sale blender keeping the mix in suspension while I coated a forge liner but a garage sale immersion blender worked well, dipping it in clean water after "stirring" the mixture.

I buttered and brushed it on my forge liner where it's been happily doing it's thing for some time now. 3 thin coats to be specific.

Frosty The Lucky.

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Thanks Frosty, for confirming straight up bentonite works. Curious as to whether or not you got a powdery coat or if it fired solid. Personally I like solid because I am heavy handed at times and I want my equipment/coating to survive that. In any case this is valuable for this discussion. (I used bentone in my mix, veegum is not easy to find here, then tweaked the ratio to suit)

Mike, have you tried any of the spheres or is this all a theory? When it comes to this I am only book smart myself :)



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Although both kinds of spheres are available online these days, I don't mix my own refractory, so far. That will probably change this summer, as I begin constructing equipment for the next book burner book.

And, while it is just book knowledge so far; I think at least one other guy on here has actually mixed the glass spheres into his refractory mix. It isn't a big leap, as similar voids have been created, employing Perlite in Kast-O-lite 30 for several years. One guy used a two-thirds refractory to one-third Perlite mix quite successfully. The glass spears should do the same job, with less increase of silicon content into the refractory during firing.

I suspect that the simplicity of ceramic wool blanket, has held off this kind of experimentation from going forward in the last two decades, but everything always changes :)

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Just to reiterate, I don't think I am breaking any new ground. I would like to have a homemade high zircon mix that holds up to lots of fire, I am not there yet.  MR Hansen himself states he has mixed results in thermal shock resistance (see https://digitalfire.com/picture/1517) And I do think that this is one of the variables in play, looking at my results. For any new forge builders that are not me; get a known refractory concrete, build around that.

That being said, and as stated before, I am pretty confident in the application as a refractory coating, as stated in an earlier post in this thread. I have yet to test it out on plain rigidised blanket but we can tackle that in this thread.

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