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Forges 101


Mikey98118

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Which brings us to what to stuff the forge with and why:
The what is various forms of refractory materials that are capable of holding up against the temperatures, combustion chemicals, and borax. But the why is no where near as straight forward is "insulation". It only takes a moment's comparison of the  heat loss through exhaust openings with heat loss through forge walls to make it clear that just insulating the forge alone is a waste of time...

You are insulating the forge to trap heat  in it it for a particular reason, and that it to super heat its internal surfaces into high incandescence; hopefully into the white hot range. Any efficient forge is a radiant oven first of all. The burner flame is primarily a heating element for that radiant heat transfer; not for heating stock directly; get that straight in your mind, or give up all hope of knowing what you're doing in gorge design.

Why? Because every choice you make about refractories, kiln shelves, and ceramic fiber products needs to reflect the need to super heat the forge interior to white heat, without rapidly gutting those materials.

That would be "forge design".

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Thanks for the clarification Mikey.  I was thinking I'd need a whole bunch of earth moving equipment if we're going to start designing gorges.

That aside, thanks for taking the time to do the burner and forge threads.  It's always good to see the "why" behind a design.

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Starting with what goes in first, let's discuss the insulation choices; that can be ceramic fiber and/or ceramic board, insulating refractory, or in the case of large forges, a secondary outer layer of Pealite. Note that insulating fire bricks didn't even make the list; neither didcalcium board, for the same reason; they can't stand up under the thermal cycling that forges put even secondary insulation must face. Also in the case of insulating firebricks, they aren't as insulating as most folks suppose...

 

ceramic fiber and the much denser board should be considered as a minimal material, which MUST be rigidized, and then coated with a at least a thin layer of heat reflecting materials in order to hold up for a reasonable time. Ceramic board holds up much better than ceramic fabric, and is well worth the added expense they make; especially as end enclosures.

Insulating refractories exchange some heat insulation for toughness, naking it well worthwhile as a primary insulating layer next to the hot-face surface. I am not speaking about physical toughness; that is a bonus.  You need to always consider both hot-face and cold-face in each coating and refractory layer, as you use both to cool the working temperatures that the forge faces in each layer of material.

the hotter you plan to run your forge the more valuable such a layer becomes.

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In most forges the burner's flame is aimed directly at the floor, or is aimed at it on an angle; the point of this is for the flame to impinge on the toughest surface that can be provided, so that other surfaces can be saved from flame impingement. High (purity) alumina kiln shelves are the most cost effective way to provide a forge floor.

Those wanting the most efficient floor possible will use small amounts of high alumina refractories in layers, starting with 3000 or 3200, then 2800, 2600, and finally 2300 refractory over ceramic fabric or board insulation. This scheme also allows the floor to be shaped, rather than flat.

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There are several kinds of materials used for hard firebricks, but only one kind is normally sold for insulating firebricks: that is the pinkish to yellowish bricks made by including including a foaming agent in refractory to make the lightweight  bricks rated to 2300 F that you see used all too often in gas forges.

To call them friable is to completely understate their fallible nature. calling them future rubble is more to the point. Such bricks are normally used as secondary insulation in things like Pizza ovens, or are used in electric pottery kilns.  Both of these tools tend to heat up and cool down slowly during very long thermal cycling; just the opposite of a forge.

Nothing stops you from using 2300 F alumina insulating refractory (which is NOT made with a foaming agent and cures into hard brick) in a simple wooden form to build your own insulating firebricks; ones that will last. So, why aren't people doing so? The commercial bricks are cheap and easy to procure; they make a brave showing...at first. You don't expect a newbie to faithfully post a follow up report on how his forge is falling apart after bragging about it, do you?

You could even make 1" thick slabs of 2300 F alumina insulating refractory  as hot faces between the foamed brick and heat source, and coat them with a high temperature mortar, and then coat that face with a radiant "mirror", and expect to do well.

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There are insulating fire bricks ranging in  from 2300 clear up to 3200; this highest  rated brick, will hard is hf the weight of a high alumina refractory and half as good at insulating as a 2300 rated brick. But, you won't find them down at Loes Hardware, which is why making your own brick from insulating refractory is recommended.

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While the strength and durability of various 2300 insulating fire bricks and/or insulating refractories to make bricks and forms from do very widely, all of them have an insulation value in an environment that is at or above 2000 F, which equals that of ceramic fiber blanket products. On the other hand the bricks can provide structural integrity, while the blanket can be shaped into forms and then rigidized into featherweight blocks of insulation.

even the cheapest grade ceramic fiber blanket doesn't melt below 3200 F. Product temperature ratings come from the level of heat the fiber products will withstand without massive shrinkage; this should illustrate the importance of locking the individual fibers in more secure positions by rigidizing; it also demystifies  the seemingly magic protection given by a relatively thin sealing coat of high temperature mortar, followed by a heat reflecting barrier such as ITC-100.

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Ceramic fiber products need both rigidizer and finish coatings to do well in today's gas forge; this is because better burner designs and better forge bodies create much higher temperatures than were common in the past. Rigidizer is especially important, because if you want your insulation to last it must be prevent as much as possible from shrinking.. On the other hand, between using 2600 F insulation and rigidizer, you can toughen the secondary inslulation layer in your forges enough so that it should stand up well to the heat that will leak past the high emissive coating (AKA IR reflector) and thin hot-face layer (typically Kast-O-lite 3000 or a high temp mortar): it also helps support the thin coating. Their are products like Plistix touted as heat mirrors which don't agree with, but which make very nice surfaces on which to paint high emissive coatings .

(A) You don't want to use thick insulation layers; instead of a single 2" thick layer, place the insulation in two 1" thick layers. Ceramic fiber blanket will easily part into thinner layers via delamination between layers.

(B) Rigidize each layer after installation, and heat cure it, before installing the next layer.

(C) Form  the burner openings before rigidizing each layer. remember to leave them just a little oversize so that they can be finish coated with a hot-face layer.

Rigidizer is colloidal silica (just fumed silica, which remains suspended in water) and common everyday food coloring (to allow you to visually judge how much to use); this water born product is easiest to dispense by spritzing. But, you can always pay though the nose for it from a pottery supply if you prefer; I buy mine through eBay and get free delivery ;)

that should read ... it must be prevented "".

Second try; that should read "...it must be prevented".

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Low cost rotary tools for steel building the easy way

I suppose that one of the biggest hurdles for the novice, when building burners, and the forges they run is what tools to buy for the job. A hand held rotary tool (preferably with accessories kit) is highly recommended. Avoid variable speed versions. Yes, it is handy to be able to vary the RPM on a rotary tool, but you want to do that trick by plugging it into a separate feed controller like those made for routers; the reason for this is that speed control, because its circuitry is too delicate when they are mounted in the rotary tool itself, and so they burn out quite easily. Single speed rotary tools cost so much less that you can buy the router control for less than the price difference, and not only have a much tougher tool, but one that has a wider speed range in the l0wer RPM range to boot. If you can’t avoid ending up with a variable speed version, you are still better off to run it at full speed and use a router control to vary its RPM; thus saving wear and tear on the tool’s own circuitry wile widening its lower end output.

Accessories have been improved even more than the rotary tools.  Cutting disks were originally made to create very thin cuts in rings and other soft jewelry items; many still are, but steel cutting friction discs have been perfected along with the spring loaded mandrels they mount on. The Dremel EZ Lock mandrel, and EZ Lock 1-1/2” cutting disks allows you to make delicate internal cuts for air openings in burners, quickly do all the cutting needed while constructing forge shells, and rapidly cut angle iron for equipment stands:  http://www.toolbarn.com/dremel-ez406.html?utm_source=google&utm_term=&utm_campaign=CPCS+-+Shopping&utm_medium=cpc&utm_content=ssXU2pY6_pcrid_90614277014_pkw_PLA_pmt_b_pdv_c_

A Set of diamond coated burrs replaces hand files; they are fast, easily controlled, economical, and unlike rotary files, they don’t fling needle sharp debris: https://www.amazon.com/SE-82331TF-30-Piece-Titanium-Coated-Diamond/dp/B000P49BX8/ref=sr_1_9?ie=UTF8&qid=1472282746&sr=8-9&keywords=rotary+tool+accessories

Drum sanders (the mandrels) and their rings/sleeves (the actual sand paper product) use abrasive sleeves to slip over an expandable rubber drum; they are hard to beat for quickly removing a few thousandths of an inch to make tubing  or pipe parts fit perfectly. eBay has the best selection of them for the lowest prices (usually with free shipping). http://www.ebay.com/sch/i.html?_odkw=rotary+tool+accessories&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H2.Xrotary+tool+drums.TRS0&_nkw=rotary+tool+drums&_sacat=0

The best design for small drum sanders uses a bottom nut for tightening, in stead of a top screw: http://www.ebay.com/itm/Drum-Sander-Kit-by-Ali-Industries-/152045632041?hash=item2366a03a29:g:Li8AAOSwuzRXfUnC

Also available from: http://www.ebay.com/itm/Drum-Sanding-Kit-No-354100-Ali-Industries-3Pk-/172300782691?hash=item281ded4c63:g:OiQAAOSw6n5XqgTj

 Also available from: http://www.ebay.com/itm/Drum-Sander-Kit-by-Ali-Industries-3PK-/162178434550?hash=item25c29685f6:g:-hAAAOSwV0RXvGce

Also available from: http://www.homedepot.com/p/Clesco-9-Piece-Super-Long-Sanding-Drum-Kit-with-Nut-Lock-Drums-DRUM-KIT-SDK-3/203930651?cm_mmc=CJ-_-1319015-_-10368321&AID=10368321&PID=1319015&SID=1083750474754&gclsrc=ds&gclid=COGX_oS14s4CFRKmfgodBqAKtg&cj=true

Rotary flap wheels also work very well: http://www.ebay.com/sch/i.html?_odkw=bottom+nut+sanding+drum&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H1.Xrotary+flap+wheels+.TRS0&_nkw=rotary+flap+wheels+&_sacat=0

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Sealing and high-emissive coatings for ceramic fibers and other surfaces

Rigidized ceramic fiber products still need to be sealed for safety. Furthermore the various coatings used for sealing tend to create a tough surface layer that holds high-emissive coatings from peeling away from the fiber’s surface; an irritating tendency that results from spreading high-emissive coatings directly on fiber products (especially those that aren’t even rigidized). Just as not all sealants are rated as high-emissive, not all high-emissive coating are sealants, so we  need to review the better known products:

ITC-100 is strictly a high-emissive coating; I have found that deliberately separating it by adding more water causes the non-colloidal particles to separate out, refining the coating, and greatly increasing its emission of radiant energy. For less money than this product now costs

100% colloidal zirconium can be purchased from various labs and mixed with phosphoric acid to make a high-emissive coating rated above 90% “reflective” of radiant heat.  

Frosty and others on this group concoct a tough sealant coating that is also a high-emissive product; they get their zirconium silicate flour for it from Seattle Pottery Supply, and mix it down with clay powder; ask them for particulars.  Zirconium silicate, while very tough is only rated at about 70% “heat reflective,” but I think this figure is misleading; since the other part of its structure is clear natural crystal, which will pass light rays with very little interference, and since the actual mechanism for its “heat reflection” is re-radiance, I believe its overall performance in thicker layers will prove to be considerably higher than 70%; it is also very resistant to borax, and an economical choice.

Plistix 900 can be purchased from Larry Zoeller Forge; it is rated at 70% heat reflection, and makes a tough smooth sealing coat rated for use at 3400°F; price is $13.00 lb., or $25.00 for two lbs., plus shipping: http://zoellerforge.com/forgeparts.html

 

Also available from Wayne Coe for $15.00 per pint (1 1/2 pounds) plus shipping:

http://www.waynecoeartistblacksmith.com/

 

Matrikote 90 AC Ceramic Coating (one of the product line from Allied Minerals) is a very tough hard coating containing 90.4% alumina, 1.5 silicon dioxide as a vitreous(glass-like) binder, and 2.7 % phosphorus oxide as a polymerizing binder. Matrikote is good to 3000°F, and would prove especially useful as an inner layer between outer coatings of higher use temperatures and rigidized ceramic fiber products; it is $15.00 for a 1 pint bag, plus shipping from Wayne Coe: http://www.bladesmithsforum.com/index.php?showtopic=22824

There are other bonding mortars and high temp coatings. Some people use 1/4” to 1/2” thick coatings of high alumina refractories.

 

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If a forge shell is really thin (stove pipe) you will have to rest heavy parts on a separate outside rest. Also, a very thin shell will not stand up under physical abuse, but then neither does the forge lining inside a thick shell.

An extra thick shell will always be a burden; literally. But the worse thing about thick shell parts, including doors, is their tendency to move out of alignment during the heating cycle. And of course thick steel parts are so easy to weld together, right? But welded shell parts are even more inclined to move around during the heating cycle.

I've been a multi-certified welder for over forty years, but every casting furnace and blacksmith forge Ive built has been threaded and/or pop riveted together; not one of them ever moved at all during heating or cooling.

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Using Silica Based Rigidizer

Ceramic fiber products are so porous that water runs right through them, unlike solid refractory, which must be slowly dried out, and then heat cured to prevent damage from build up of steam pressure. So, ceramic fiber can be "cured as you go," which means that nothing prevents you from slowly rotating an outer layer of blanket on a curved surface, spritzing the rigidizer to each area that is laying flat under the weight of the liquid, using your burner (turned down low and constantly moving over the wet fiber), to stiffen  the blanket into permanent shape, and then moving on at your convenience. After creating a smooth stiff surface inside the structure, you proceed to install the inner layer the same way. One of the joys about completely soaking the blanket through, is that both layers will bond together. Any rigidizer that soaks into the outer layer will run right over its fibers surfaces by capillary action, the same as it did the first time, causing no build up to degrade the insulating value of the outer fiber layer.The whole process is mostly idiot proof.

I did say mostly idiot proof; if you dream off, you could burn yourself from the escaping steam that will be created. If you turn a really high speed burner on at maximum while totally dreaming off, it is conceivable (but not likely) that you could even melt a piece of fiber. Finally, someone on drugs might be stupid enough to point the burner at himself...

What is there to keep Murphy from messing up your efforts? Firstly, the fiber is partly alumina, and partly silica; the alumina pretty much prevents it from melting, while the silica content bonds beautifully to the colloidal silica in the rigidizer. Secondly, the individual fibers in the blanket are very thin, which maximizes capillary action of a liquid across a surface. During heat curing, the colloidal silica that has wet every surface becomes a permanent vitreous outer layer on the fiber, which creates welded joints everywhere the fibers cross each other. This glass sheathing is permanent. More rigidizer applied over it simply adds another layer after the next heat.

What can possibly go wrong? Well determined idiots will be glad to note that, if you don't completely cure the rigidized blanket before coating with sealant,it can still create a steam pressure problem.

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Yes, TFT.

Ceramic fiber products transform many of its physical characteristics do to repeated heating cycles; none of those changes are good. Rigidizing slows down the damage.  Also, you want a hot-face inner layer that holds the sealing and toughening layer to physically support the high-emissive coating, which wouldn't last very long without it. Well, rigidizing the insulation behind the hard coating helps support it in the very same way; why wouldn't you want to do that too?

 

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On 8/28/2016 at 1:34 AM, Mikey98118 said:

ITC-100 is strictly a high-emissive coating...100% colloidal zirconium can be purchased from various labs and mixed with phosphoric acid to make a high-emissive coating rated above 90% “reflective” of radiant heat.  

...since the actual mechanism for its “heat reflection” is re-radiance,

...this has confused me for quite some time. I keep reading about ITC-100 etc.. being highly IR reflective, yet when I try go and do some research I find they have very high absorption and emissivity instead.

So to put the matter straight in my head: Reflection has nothing to do with this - it's all emission? An "ideal" forge interior coating will get very hot (via IR absorption plus conduction) and will then emit lots of IR. That emitted IR is doing the bulk of the work heating the workpiece. Is that right? Using the term "reflection" might make for easy understanding, but is not actually what we're dealing with....?

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That's pretty close to how it works Andy. High zirconium products aren't very good conductors but they do collect heat like thermal batteries and re-radiate it. It goes everywhere though so it's a good idea to have a layer thick enough it's low conductivity tends to keep it aimed at the forge. No, don't quote me on that, I'm sort of repeating what I was told by a fellow who uses zirconium kiln washes and was a research junky.

When ITC-100 first came to the attention of the blacksmithing community via the internet it was mistakenly dubbed a "black body IR reflector" and it's nothing of the kind, not even close. It's tougher than any propane burner we can make or flux we can come up with can touch. The stuff is darned near diamond hard and almost inert.

So, anyhow your research is correct, the whole reflectivity thing is a misunderstanding that has become a blacksmith myth.

AS it's an IR absorption and emission product a really thin paint on or spritz on coating is ineffective. ITC-100 is designed to protect surfaces from hot HARSH atmospheres, NOT heat nor make the furnaces more efficient. Large commercial furnaces use an entirely different technique to make them more efficient, a "recuperative wall" is a main technique but that's a whole different kettle of noodles and not so applicable to the home forge.

An effective kiln wash in our gas forges needs to be pretty thick, I'm shooting for 1/4" on the forge floor and flame impingement surfaces over a hard inner liner. I'm currently experimenting with small coupons of a Zircopax flour and Kast-O-Lite 30 LI mix. I'll report my results when I'm done. I just don't want an inch of hard liner soaking up heat in my forge and may be wasting a lot of time and effort.

Well, didn't what should've been a simple answer turn into a ramble.

Frosty The Lucky.

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Frosty,

Everything you said was correct; but, you have actually understated your case.

With a really high quality zirconia coating; that is to say a coating made with colloidal zirconium oxide, you get re-radiance of infrared and the total visible light bands of over 90%; then a thin coating will both re-emit radiant energy back into a furnace or forge and help pass radiant through a crucible wall; such thin coats should be made with a made with one of the expensive type of zirconia, and under .0004" thick. For something like a furnace or forge interior, the coating should be as thick as you find will work, as each layer of coating will also act as super insulation. I wrote all of this out in detail a few months back, but can't find it on the old pages anymore.

That should read ".004".

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Kiln shelves are mostly considered a topic of interest to potters, because they are normally found in electric kilns. What goes on in these kilns that heavy heavy loads of pottery are cured at high heats for several hours; which is why kiln products, including shelves, are rated by cone numbers, rather than merely by rated use temperatures. The bottom line is that these shelves are engineered for maximum resistance to slumping under loads at sustained high temperatures—not for insulation value, which for kiln shelves is considered counterproductive. In fact, high alumina kilns are the perennial favorites of small business users despite their minor insulating value; not because of it.

    The most expensive kiln shelves are  nitride bonded silicon carbide, because of their high loading capacity and low shelf weights, which are important factors in a pottery kiln. Both carbon and silicon transmit heat well, but that is a plus factor—In a pottery kiln; not in a blacksmith forge or casting furnace. Fast heat transfer, combine with a maximum use temperature of 2600 degrees makes this a less than stellar choice.

    Mullite kiln shelves are made by fusing magnesium and silicon together; a use temperature of 2900 degrees compares well with high alumina shelves; it is noted for thermal shock resistance, but high alumina shelves or very good at that also. Mullite is a poor insulator, and not as strong as high alumina, but the product is an acceptable alternative to shipping costs, if high alumina isn’t locally available.

    Half bricks (1” thick fire bricks) are only acceptable for whose who’s common sense is blinded by parsimony.

    A floor made of semi-insulating high alumina refractory, over ceramic fiber insulation, is an acceptable substitute for a high alumina kiln shelf, and perhaps even advisable if you want it shaped; but it is nowhere near as strong as any kiln shelf. As is often the case, once you have a looked at the facts, choosing between viable alternatives can be hard.   

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  • 2 weeks later...

Vertical tube forges, and casting furnaces are a glaring exception to the usually desirable technique of quickly slowing a fast flame down, to increase its hang time in the equipment. Will swirling the incoming flame is used both in vertical and horizontally positioned tube forges, and in small casting furnaces, The flame in vertical equipment should always be encourage to run as fast as possible, in order to resist being sucked out of the exhaust too fast, do to buoyancy.

It is desirable to slant a burner slightly toward the exhaust opening if is placed nearly to the closed end of a horizontal forge. While the burner in a vertical forge or casting furnace should always be near to its bottom, it should also be slanted slightly downward (away from the direction of the exhaust opening), to help fight buoyancy.

Doing these things only slow down the effects of buoyancy for seconds, but even a few extra seconds will allow the flame to transfer a lot more heat into the equipment and the work it is heating.

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Why, yes; my best advice is to look into some of what has been written on small furnace design in various casting groups over the last few years, because it would be hard to find a better primer for building a tough efficient vertical forge than some of the designs for various small casting furnaces. When, you feel ready to ask various "yeah, but don't you think..." questions, we can iron them out  right here (sorry; couldn't resist).

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