7 gallon perlite/sodium silicate/aluminum oxide forge lined with 3000 F refractory

[Jeff Reed]

So, I am a newbie...both to the website and to forge making...which may mean my first attempt at building one could be called a "forgery". Sorry. Couldn't resist. Admittedly, there are a lot of great ideas on how to make a forge. Seemingly endless, actually. I decided early on that I was as interested in the process of making this, as I was in "making this".  It HAS been a process.  Everyone has seen the video on making a small forge with a stainless garbage can using perlite, sodium silicate, and aluminum oxide. The design has drawbacks...they all do.

So I played around with making a much bigger version out of an old 7 gallon propane tank, but kept the internal diameter at 3.5". Two burners rather than one seemed reasonable. The wall thickness of the forge body was thereby increased relative to the cavity. It took a heck of a lot of perlite and liquid glass to fill, but casted nicely. It ended up using nearly 3 liters of sodium silicate, so thank god I had a bunch of NaOH. The inner cavity is lined with refractory cement rated at 3000 F. The burners are 1/8 stainless, with a bell I made out of a drift I had previously hammered using a coal forge we built from a brake drum. The mig tips I used are 0.035, and when fed by the 30 psi high pressure propane line, throw off a big hot blue flame.  Both ends of the forge are open for now, but can be closed if desired. 

We tried it out with good results. A leaf spring heated cherry red in 2.5 minutes. The burners are recessed into the forge body, and the 15 degrees off center placement gave a spinning vortex in the chamber. Once I get a heat gun to measure temperatures, I can relay how hot we got, but for now, the example is all I have. As far as durability...yet to be seen. 

Other than making fun of my choices of materials, I would like to hear what others may think or advise... has anyone made a big perlite/sodium silicate forge? If so, what were the main problems downstream? 

 

Thanks, Jeff

[Frosty]

Welcome aboard Jeff, glad to have you. If you'll put your general location in the header you might be surprised how many of the IFI gang live within visiting distance.

I'm unfamiliar with building forges the way you did. Do you have a link to directions or a video so I can get a handle on what you're talking about.

Your description lacks the kind of details we'd need to review what you have. Can you post a Youtube video of your forge running and maybe pics showing the details of it's construction especially the burners.

Frosty The Lucky.

[Jeff Reed]

Thanks for the kind welcome, Frosty!   

This is the video I referenced about constructing the forge body from a stainless steel liner, perlite, sodium silicate and aluminum oxide.https://youtu.be/vQN7EqGMTuo Again, it is roughly 1/5 the size of the one I made from the 7 gallon propane tank. The sodium silicate, or "liquid glass", isn't readily available, so I made my own. The source of silicate gel was Hobby Lobby, and was originally sold as a flower desiccate product. The quantities of NaOH and silicate gel are 200gm and 300gm respectively, and produce an impressive exothermic reaction when mixed. I pushed the reaction to completion with an humble propane camp stove commandeered for the project. It made a good 2nd choice for the bunson burner I didn't have. The same author for the forge showed how to make the sodium silicate in a different posting. I improved on his technique by using a finer granule of silicate gel, and a stick blender to mechanically expedite the mixing. 

My apologies for the details not being what you usually need. I will grab some of the picks and make a higher quality video of the forge in operation. The burners are a version of the design posted by David Hammer in his video  https://youtu.be/VxzdqcPzXj8.  The changes I made were simple substitutions. Rather than schedule 40 3/4 inch pipe for the burner bodies, I used 1/8 thickness stainless with 3/4" internal diameter. The MIG tip I chose was 0.035, and he used 0.025. 

Hope that helps some. I will get back with the pictures and the better quality video of the forge. 

Thanks. Jeff

[jumbojak]

Crystal cat litter is a good source of silica gel. Generally cheaper than products labeled as a dessicant too. I use it wrapped in pieces from an old tshirt in my toolbox to help fight off rust. You can also buy sodium silicate by the gallon at some big box hardware stores. Apparently brick masons use it. The gallon jugs are pretty expensive, but it's a good deal safer than dissolving silica gel in a hot alkaline solution. Heck, it might even be cheaper in the end, if you continue experimenting with this type of design.

[Mikey98118]

Jeff,

I like the choice of  a stainless steel trash can for the shell; that was clever. His choice of homemade refractory and finish coat was clever too, and I wish it would work, which it will for a very short while, and then it will fall apart. Sodium silicate fails around 1900 F, and that is what is holding everything else together. Perlite also fails at about 1900 F; together sodium silicate and Perlite make a very good outer layer, IF they are protected by an IR reflective coated castable refractory hot-face (inner layer), which is then surrounded by a thick secondary layer of insulating refractory, such as ceramic fiber.

[Jeff Reed]

Thanks Jumbojak. I would have never thought of the cat litter!

Mikey,

Thanks for the help. The inner layer of the forge is coated with refractory, as I was afraid the perlite/sodium silicate/aluminum oxide would fail. However, I was hoping that the refractory layer would be sufficient. I will take your advice and look for a good price on ceramic fiber...do you have any insight as to which source is the best value? 

One more question, after the ceramic fiber layer is applied, will it also need to be finished with an additional layer of something like ITC-100? 

Once again, thanks for the help.

Jeff

[ThomasPowers]

Without a location; sources are useless to be "best value".  If you live in Russia sources in South Africa are probably of no use to you.  Over 100 countries participate in these forums---why Frosty suggested you edit your profile and add a general location in the first reply to your original post.  Or you can just include it in every question that has a location component. (for example I am posting from Mexico right now!)

[Mikey98118]

Jeff,

Jeff,

I can see that you're into taking responsibility for understanding and using "raw" materials, so yes, there are a number of choices that beat the heck out of ITC-100; not only costing less money, but giving better performance at the same time.  And since  a so called IR reflector (actually a high emissivity coating) will be of especial help in your situation, let's lay them out.

In the first place, ITC-100 claims "up to" 90% IR reflection. And, you guessed it; "up to" is actually a cover for the nasty truth that their formula can also mean "low as" 67% reflection; its all a matter of particle size. Being a naturally suspicious cynic, I tried separating the colloidal content from cruder particulates in their product by spooning some of it into a glass of water and presto; the crude stuff fell out of suspension and immediately sank to the bottom of the glass. So, I mixed in as much as I could, and painted the much thinner coating over an already ITC-100 coated surface. My forge went from bright orange to yellow-white incandescence with the same burner and regulator setting. Point proved.

When ITC products first came on the market, stabilized zirconia cost twice as much as the regular kind. Today, there are three different ways to stabilize zirconia, and the price has fallen to about one-third more than the regular stuff; this is an important factor to keep in mind.

So, if the colloidal stuff is so much more effective why have crude particulates in the content? MONEY; what is commonly called zirconia "flower" is nearly 100% colloidal, and will give you the full emissivity benefit; but it's not cheap. Why, did you know that a 500 gram bottle of this stuff will cost a full third of the price you are paying for ITC-100?!? But, wait, the ITC product has water content, and a SECRET INGREDIANT!!! Obviously, you can't get along without their miracal product...or can you?

Zirconia changes its crystalline structure at about 900 F, from square to hexagonal and then back to square again during cooling, so twice a heating cycle, it also changes particulate size, which is very hard indeed on every other ingredient in a rigid refractory coating; as in turning them to dust; and so manufacturers of high heat crucibles (and others who's products justify the added expense) employ stabilized zirconia.

A product that became popular while waiting for reasonable stabilized zirconia prices to happen is zirconium silicate; since this has silicate and zirconium mixed in a crystalline structure, it makes an end run around the problem, and is reasonably priced. You can buy zirconium silicate already mixed with a binding agents (Zircopax?) Both zirconia and silicate are very resistant to flame erosion, and chemical attack, and are generally tough ingredients for a hot-face coating. however zirconium silicate separates out into its two constituents over time at high heat, and it is only about 67% IR reflective.

So, if you want maximum protection for your hot-face layer, stabilized zirconia mixed with a good refractory binding agent (ex. calcium aluminate) is the optimal path.

To continue, the concept of high-emissivity coatings rather than using the technical shorthand phrase "IR reflector":

Yes, there actually are substances that reflect infrared energy; the most notable being gold, followed by silver and aluminum. But the difference between cause and effect is important. Actual IR reflectors are only useful as ultra thin coatings on optical devices, such as light filters in welding helmets.

High-emissivity coatings can be used to more effectively transfer heat through a crucible wall (as a thin coating), or bounce deflect energy and also insulate surfaces against heat gain in thicker coatings; to illustrate the importance of the point, we will define thin zirconia coatings as one millimeter (just under .040") and thick coatings as three to five millimeters and up. And the higher the heat level the more effective high-emissive coatings become, while every other form of insulation loses efficiency as heat levels rise. And the thicker the coating the greater the insulation becomes. Induction "furnaces" for instance, use crucibles made of zirconia refractory, which is transparent to EMP waves, but is so efficient as a high heat insulator that with secondary insulating refractory between the coils and an inch or less of it beyound the coils, becomes the furnace as well as crucible.

[Mikey98118]

So, getting back to ITC-100; what is the secret ingredient? Officially, I don't know. But government sponsored experiments with zirconia coatings back in the sixties tried a number binders; the most successful was  orthophosphoric acid (commonly called phosphoric acid); a readily available and inexpensive substance that stays suspended in water, and has some interesting physical attributes. When painted unto a surface it is adhesive, and will hold zirconia particles suspended on that surface. When heated, it polymerizes as the acid forms esters. Thereafter it remains on the surface in a vitreous form at normal temperatures, and becomes adhesive above 365 F from then on. Does this remind users of ITC-100 of anything?

Well, there are your options, and every one of them is better than using the ITC product, yes?

[Mikey98118]

On the other hand, is 67% heat reflection such a poor showing? YES it is, which is why products like Plistex can legally claim to be "almost as good as ITC-100," but the truth is that what's legally claimed and what's true don't match up. You see high alumina products are such poor  heat conductors that they are rated at 70% heat reflection. However the  67% reflection of crude zirconia particulates has to be averaged out with the "up to" 90% heat reflection of the fine particles. Still when one company plays loose with the facts, it can hardly complain when its competition "puts the shoe on the other foot"

[Mikey98118]

The way a high-emissive coating works, is that it absorbs heat so easily that it quickly becomes incandescent. Think of a thin layer of zirconium oxide molecules exposed to a high heat source, and radiating that energy in all directions; now picture another layer of molecules next to the first, with still other layers behind them. Each layer radiates heat in all directions, but the heat source only comes from one direction, so at every additional layer some heat gets subtracted as it is radiated back toward the coating surface. Thus a thin coat is transferring lots of energy through a crucible wall, while the portion of heat it radiates back into the equipment is then re-radiated back at it, while the thicker coating on equipment surfaces reduces heat transfer that would otherwise happen through conduction by radiating a higher percentage of it back into the equipment interior.

So, high-emissive coatings are a simple but elegant form of recuperative energy generation.

[Mikey98118]

As to the other question, local potters' supply stores usually sell ceramic fiber blanket, as do boiler shop and HVAC suppliers, and  some stove supplies companies. If you can't find what you need locally, or can't find it in small enough amounts, Wayne.co will help you. McGuills Warehouse is usually the cheapest online source for full roles, but they are located in Southern California. Since distance affects shipping costs, how far you live from them versus some other source also comes into play.

[tonyw]

Fantastic write up Mikey! I love it! I think it is fascinating how all this works, and I really appreciate you explaining it. Does your book have more stuff about and like this? Perhaps I will ask for a copy for my birthday.

[Mikey98118]

The published book has lots of USEFUL technical information, but not on this subject. Probably the most valuable information in it was on equipment building, although people usually buy it for the burner write-ups. Outside of the multi-hole, which caused a sensation for the first couple of years, I've heard little on the other equipment, directly; although the forge has appeared in various altered versions on youtube. You need to remember it's been published for nearly twelve years, and we've all moved on, yes?