Forges 101

[Mikey98118]

The beginning of forge design is the study of burners. The proper end point of burner design is the equipment they power. Lumped together, these subjects are too unwieldy; at the same time, they can't  stand-alone, since their study is as intertwined as the equipment itself.

What kind of forge is best is an arbitrary  question, without any adequate answer. But, whatever design you favor, there are common factors which lead to a desired end point--or away from it.

Size: Bigger is never better. once a forge is outmatched by the work load it becomes a part time tool. But, an outsize forge quickly becomes an embarrassment collecting dust in a corner.

Shape Is mostly a question of personal preference. You can always tell how personal by how loudly this view is denied.

Exhaust size and shapes: Like so many other things NO size or shape is ever quite right. Therefore V-A-RI-A-B-L-E is the proper size and shape; all others can be outright wrong, but are never just right.    

[Mikey98118]

Finally, in forge design one half the utility of a forge comes out of its design; the other half rests solidly in how you execute that design. Muff ether part, and you just create a mess.

The shell is where most people start their figuring from; it does more than hold the forge materials to together; it is far more important for a handy place to rest burner ports, exhaust doors, and legs. Furthermore, nothing stops you from using the forge materials to help brace the shell from its inside.

To begin with, the shell only needs to be thick enough to provide an easy surface to mount parts too, at a minimum. Of course a few extra thousands in the shell wall can save you a lot of work, and make the structure more rigid. A 1/4" wall pipe is way too thick; think 3/32" wall, tops. But, stove pipe is a lot thinner and some guys use it quite happily with sheet metal crews or pop rivets.

Some guys hinge one or both ends of the forge. in order to get better control of parts, and to make construction and repairs easier. Other guys hinge the top and bottom halve of the forge together; I don't, so far...

That would be"either" part.

[Mikey98118]

Shape is a matter of opinion and we all have one; some of us have way too many  (yes, I plead guilty as charged). The most popular shape around is the tube forge. Why, is probably because so many people have built one, that they have become the proverbial "well worn path"; they are also out of date. Oval shapes have been around for more than twenty years, and are finally catching on, because they are a real improvement on tube forges.

I suspect that oval forges started out as a way to get more use out of forges than the tube shape could give. But, as burners have become hotter, the added room before your flame impinges on Kaowool has become increasingly important. Most people face their burners down at an angle, so that they impinge on a high alumina kiln shelf floor; these are cheap, easily replaced, and very tough; non of which can be said of ceramic fiber materials. The floor area in an oval forge will end up at least one-third wider than in a tube forge...

The slickest forge design that I have ever seen is an oval mini forge built from half a car muffler, which can be found on an older thread in this forum. Larger oval forges require sheet metal work.

[Mikey98118]

The problem is that first timers get hung up with things like "the easy way" that usually isn't, and "big enough". Big enough for what; for some nebulous plans for someday when they are rich and famous?Getting rich and famous "someday" Kind of requires paying attention to what your doing this day. All the rich and famous artists tell various versions of the same basic story, which goes something like "I just got into this thing as a hobby, and it kind of got totally out of hand; so that's how I got here." Can you see the obvious? It wasn't some secret knowledge or virtue that they followed, but taking time enough to avoid all that fly paper the eager beavers end up stuck on that got them to "here". Take the time to understand WHY thing are made the way whey are in good forge designs; otherwise, there is a whole world full of fly paper waiting eagerly just for you. 

All this to say " make that first forge small".

Every part of a good forge has an important purpose; no what it is before you start designing yours.

[ThomasPowers]

I get a lot of "Propane forges do not get as hot as coal" to which I give them a blank stare and say "I've melted steel in this forge before; how much hotter do you need it?"  (First time you melt a piece it's  "NEAT!"  Very soon it's a "Not Again!  When will I learn not to leave a piece in when I answer the phone...?" type of thing.)

[Mikey98118]

Thomas,

Isn't it amazing how you can make such a significant leading statement as that, and be totally ignored by the willfully ignorant? Ever feel like you're stuck making a movie with the Three Stooges?

Exhaust openings:

One thing the backyard casters and black smiths both worry over is how large to make the exhaust openings on there equipment. Too small and you have high back pressure killing burner performance; too large and you can't get enough heat to stay within the equipment interior to do the work. Of course the closer to the "right" opening size you make the stronger the equipment can be built. Just don't get suckered in to confusing the right size for the perfect size. So long as burner out can by varied (turn-down range). There can't be any such thing as a perfect size. The right size is what is needed to accommodate the burner's highest output (the highest you are willing to take it to). If you want best performance at lower pressures, they can easily be provided be forge doors that are variable baffles.

How to do this unless you already have a r-e-l-i-a-b-l-e figure to start from? You need to make up something with an exhaust hole on it that can be varied in size to play with; try brick.

Even when you get the best performance from your burner with the exhaust wide open,  you  are likely to be fighting a cold spot from doing so, try separating exhaust losses from radiant heat losses by putting up  a movable barrier of brick or a drilled kiln shelf, at a small distance from the opening, to allow exhaust gases to move up and out, while bouncing radiation off of an ITC-100 coating, and back into your forge. Move the barrier closer and farther away from the forge opening, depending on how high you are running the burner, and otherwise keep the hole in the brick  only as small as needed to move the parts through.

[Mikey98118]

 

This arrangement helps to slow the flow of expended gas in the forge interior, even as it heads toward the exhaust opening; and then speed the gas up at the opening; another highly desirable trade off, but how exactly is this trick done? As the gas exits through the restricted area of the exhaust opening its flow speeds up do to bouncy, much as a river's water speeds up as it approaches the falls. But isn't any opening subject to buoyancy? Yes, but the less its restriction the less flow speed is slowed down in the forge interior, and the less it is sped up at the exit.

So, you are gaining hang time for the heated gas in the forge, and and recuperative savings from bounce back of radiant enery; a win-win situation.

[Mikey98118]

Is there any improvement to be made over fire brick as the "baffle"? Yes, but only after you use the forge enough to know what size and shape opening you normally favor: even then you will want to keep the brick on hand for use with unusual parts.

High alumina kiln shelves are seven times as insulating as hard fire brick; it is also tougher at heat, which is an important consideration for something you will end up setting parts on.

Using alternate baffles, with different openings, or building an elaborate system of moving kiln shelf parts to ape the ability of bricks to change their openings comes under the heading of "gilding the Lilly." The additional energy savings it provides probably isn't worthwhile.

[Buzzkill]

You may have covered this before, but it came up in another thread so I thought I'd ask it here:  For the purposes of calculating forge chamber volume to determine the size and/or number of burners needed to properly heat the forge, should you deduct the volume taken up by things such as fire brick or kiln shelf?  I know they are initially heat sinks to some degree but then they help radiate heat back to the steel, so I'm not really sure how to treat them in those calculations.

[Mikey98118]

I would; it doesn't take long to bring a 3/4" or 1" thick high alumina kiln shelf to match any other surface temperature you will find in the rest of the forge. On the other hand, that sure wouldn't hold true if someone is using a half brick for the forge floor. Also, you want to coat the shelf with one of the Frosty kiln wash formulas; I used to use a modified ITC-100 coat, but I believe Frosty has the better idea for a wear surface.

[Frosty]

Yes, the gross number to calculate the ratio is the open volume. The insulating and heat sink properties of the material the chamber is made of effect things as well. A hard fire brick forge floor will bring a forge that should hit high yellow, melt your project if you aren't paying attention, to medium orange so so forge.

So far my results using a sifted castable refractory as a binder for Zircopax, (zirconium silicate) flower has been so so, so far. My next experiments will be mixing Zircopax and Kast-O-Lite 30.

Frosty The Lucky.

 

[Beatsy' s Forge]

Frosty, 

    Have you ever used any product from vesuvius called stick tight it is a mix of K-Wool and fire clay. I used the stuff to insulate and fill holes in refractory at US steel for a couple of applications it worked great .The only thing about it is after it drys it cracks,I wonder now if it were mixed with a stronger binder if it would hold up better and perhaps be used in a lining application. Also I use to work with some light weight gunnite that was nice,and they also make a pump able version of K wool and fire clay it comes premixed in 5 gal buckets it was at the time called pump able. Also do you use stainless needles to keep the castable held together? We used them when we grouped up takes at the Electric Furnace at the AK steel plant in Dearborn, Michigan. And rather than regular hard firebrick has anyone used black brick for a forge? I haven't seen it yet but we used these for the hot linings or working linings in furnace, vessel, steel laddel and other applications only sometimes did we spry gunnite on it and that was when the molten steel/Iron would have to sit in a laddel or tread well and a few other applications but funny as it may seem we never put gunnite /shot create on the working lining of a blast furnace or vessel only  erythromycin long black brick with a hard yellow brick backer and we're the yellow brick had a void between the brick and the shell we filled it with dry pack castable or ram .

[Frosty]

Nope, I've never used any of those, I'm a hobbyist with one too large home shop forge not a commercial operation. Is what you're calling K-Wool "Kaowool" or something else?

The stick tight sounds like a patch material but how does it insulate? The insulating properties of ceramic blanket is two fold, the material itself is a poor thermal conductor being largely silicate and secondly by the trapped empty space between fibers. What is stick tight's consistency? I doubt it's something I have a use for but you never know, I have Kaowool scrap and fire clay so who knows.

The rest of the products you refer to is way out of my league.

Frosty The Lucky.

[Man_On_Fire68]

Hey guys, I've been lurking these threads for some while now. I'm a relatively new hobbyist bladesmith, as I have been forging/researching knife making/blacksmithing seriously since about January. I've made about a dozen successful knives and various other things using a great coal forge, however, my coal supply is dwindling and living in northwestern Ontario, it has been exceedingly difficult to find a coal supplier. Thus, I have begun to seriously consider constructing my own one burner propane forge specifically for knife making and some longer pieces like machetes or perhaps a eventually a sword. My big question is: what would be optimal for these purposes? Since in my experience and research it is only wise to work about 5-6 inches of hot steel at a time, and propane forges will heat as much steel as they are long, would it not be best (as far as fuel efficiency goes) to have a forge that is only 5-6 inches in length?... With one burner?... Even for pieces that are exceedingly long, like a sword?...Unless you're heat treating said exceedingly long piece?
  Anyways, I have spent the last week on my free time (between working 50 hours a week and spending time with the girlfriend), pooling for hours over all the information available on this website available on propane forges, their designs and their efficiency. Some of the information is conflicting, some people disagree on certain areas and designs. What I am most confused about is the exact process of lining your forge in a manner that will maximize efficiency. My understanding is that there are varying degrees of qualities where refractories are concerned (you get what you paid for). 

This is how the lining of a forge is layered from outside shell to inside (from what I understand)
-The shell is relatively thin I was thinking for my shell, I would use 14 or 12 gauge mild sheet steel
-ceramic blanket 1''
-castable refractory 1/2'' (preferably high alumina content?) (how exactly is this applied to the blanket?)
-thermo-reflective coating? is this a thing? is it necessary?


I greatly appreciate any help and advice I can get with the design of this forge, I simply want to make sure I understand how the lining is designed to maximize efficiency while also being durable so I can get the longest amount of forging time without having to replace it too frequently, before I go buying things that I may or may not need

Thanks guys,
Zach

[natenaaron]

Go two inches on the blanket

[Frosty]

You have it about right Zach. I don't know about making a forge quite that short but it should work. The problem with a "reverberatory" forge this short is how they work. The flame doesn't heat the stock directly, sure it pumps some heat in but the main heat transfer comes from the flame heating the forge liner and the liner heating the stock by radiatiant heat.

So, the stock at the ends of the forge will not get as hot as that inside, even a little way. If you want to b able to work say 5-6" then I'd add a couple inches at each end. I think 10-12" would work better though 7-8" might do the trick. I've never experimented on this line so I'm making educated guesses based on my general experience with gas forges.

I prefer 2" of backer, the insulating outer liner and I prefer Kaowool or the equivalent ceramic blanket refractory. The flame face or hard inner liner is definitely going to be more durable if you use a high alumina refractory. Welding fluxes are mostly based on borax which at welding temperatures is caustic and caustics dissolve silicates. The more common refractories, ceramic blankets included are silicate compounds and do NOT like welding fluxes, blanket dissolves like cotton candy under hot water.

There are a number of ways to for the hard liner you can cast it then wrap it in blanket or mix the refractory like wall Spackle and trowel it on. You'll want to use a rigidizer on the blanket first to reduce it's flexibility. You don't really want a thick inner liner, the less heat sink the better, you don't really need to spend fuel just to keep the rock warm. Well, not more than necessary, a little thermal mass is a good thing.

The IR "reflective" layer is actually a high efficiency re-radiating material rather than a reflector. It works just like any liner in a reverberatory furnace, it gets HOT then radiates the IR. It's just really efficient compared to hard refractories. ITC-100 is based on Zirconium silicate in a kaolin clay binder but it's pretty darned expensive. Plistex and Metricone are similar products and much less expensive. Check with Wayne Coe for smaller quantities than the manufacturer wants to sell you.

I've been playing with varying results with mixing a % of Zircopax (zirconium silicate flour fro Seattle pottery supply.) directly in the first 1/2" of the hard liner refractory. I'm still working on how much I can add without making the refractory too weak. Zirconium is pretty impervious to whatever high temp chemistry we might have happen in our forges, it's almost inert. It has a vitrifying temp near 6,000f and is near diamond hard.

You don't need to do what I've been doing with Zircopax, I'm a born tinkerer and like laying with things, you can buy proven products that work. Yeah, I'm reinventing wheels all the time, I certainly didn't invent the jet ejector type burner of which a "T" is a simple variant.

Remember whatever size you make your forge the internal volume is the main factor to determine burner size or number.

Frosty The Lucky.

[Beatsy' s Forge]

Frosty, 

    Have you ever used any product from vesuvius called stick tight it is a mix of K-Wool and fire clay. I used the stuff to insulate and fill holes in refractory at US steel for a couple of applications it worked great .The only thing about it is after it drys it cracks,I wonder now if it were mixed with a stronger binder if it would hold up better and perhaps be used in a lining application. Also I use to work with some light weight gunnite that was nice,and they also make a pump able version of K wool and fire clay it comes premixed in 5 gal buckets it was at the time called pump able. Also do you use stainless needles to keep the castable held together? We used them when we grouped up takes at the Electric Furnace at the AK steel plant in Dearborn, Michigan. And rather than regular hard firebrick has anyone used black brick for a forge? I haven't seen it yet but we used these for the hot linings or working linings in furnace, vessel, steel laddel and other applications only sometimes did we spry gunnite on it and that was when the molten steel/Iron would have to sit in a laddel or tread well and a few other applications but funny as it may seem we never put gunnite /shot create on the working lining of a blast furnace or vessel only  erythromycin long black brick with a hard yellow brick backer and we're the yellow brick had a void between the brick and the shell we filled it with dry pack castable or ram .

Frosty 

     Yes K wool is in deed Kaowool, this is what we called it at work. Anyway  stick tight is essentially ground Kaowool and fire mix it is a patch but I  suspect it may work as well or better than Kaowool byitself.the other thing I am curious about is we used stainless steel needles is shot create and grouting and several other applications If you were to use a castable for the working lining of a forge no matter if it is coal or propane would hold the refractory from cracking as easy. I have castable but haven't used it yet for a forge. My idea is to use 2 in. of blanket about 4 in. of castable for the working lining. The only thing is I don't know how reflective the materials are if I  would need to coat it with something on top of the castable. 

Martin 

[Man_On_Fire68]

Thanks Frosty. So just to clarify:
2'' of backer
1/2'' of castable
and a thin layer of IR reflective?

What about the hole in the forge liner where the nozzle of the burner enters the forge? Would you lay down castable and IR reflective around the circumference of the hole right to the shell? 
 

[Frosty]

I don't put Zircopax in the burner port. I find the flame doesn't really start generating heat till it's at or near the face of the inner liner. I've had varying results trying to cast refractory into nozzle flares and shapes. A simple cylinder about 25% larger diameter seems to work well and I do kiln wash the burner port, just no Zircopax.

Frosty The Lucky.

[Mikey98118]

 

There are various high end refractories rated for 4000 F and up; none of them seems to take the heat cycles encountered in forges well. So far the only materials I've seen that do are high alumina kiln shelves and refractories rated about 3000 F; this is hot enough to last well under the hottest propane/air flame.

[Frosty]

Same here Mike, I haven't found a really high temp refractory suitable for a home use forge since Pyramid got bought and stopped selling their "Super" line of phosphate bonded hard refractories. High alumina seems to be what a person can get for less than ridiculous $.

Frosty The Lucky.

 

[Man_On_Fire68]

So, couldn't you just buy raw alumina and mix it in with a castable refractory mix? 

[Frosty]

I don't want to sound curmudgeonly but you should think about reading about this stuff before suggesting things.

The case in point: We are talking about a high alumina castable refractory, one that has been getting talked about quite a bit recently by guys who use it. Now you ask if we couldn't buy raw alumina and add it to a . . . high alumina castable refractory? To what purpose? The refractory has been formulated by experts is an industry standard product that exceeds our needs and is reasonably economical. Think adding more inert material to the recipe might, just MIGHT make it physically weaker, maybe stop it from setting up at all, or . . . ?

Did you read more than the last post or maybe three? No wait, you aren't asking about the kiln wash or are you?

We like ideas, the more the merrier but please, PLEASE get an idea of what folks are talking about before jumping in. It'll do two positive things: first we won't have to listen to a bunch of other guys who have ideas but no clue and secondly YOU won't look so silly.

It'd be a win win for us all.

Frosty The Lucky.

[Man_On_Fire68]

Okay. Thanks for clearing that one up there, Frosty.

[Mikey98118]

When looking at a flame out of the  flame nozzle from a really hot naturally aspirated burner , in a cold forge, it will look much as it does out in the open air, but within moments it will lengthen and become smoother in outline, as the forge starts to super heat; it will also lighten in hue. There will be very little secondary flame within the forge, even will it is cold. lesser burners will make more complicated flame envelopes, but this is the ideal.

First, you need to remember that there are at least two different flames going on in a gas forge; the flame being input by the burner, and the possible output flame leaving the forge via the exhaust opening. When guys discuss terms like dragon's breath it is the exhaust flame they are speaking of, which is a very different animal than the incoming flames from a burner. Not that both flames aren't equally important, but they need to be treated separately for clarity.

So, if we are speaking about the burner flame, straight blue from a total primary combustion envelope is desirable, but many older burner designs have a white inner flame ahead of a blue secondary flame, followed by a darker larger and less substantial appearing Tertiary  flame of  "secondary combustion"; BUT by that I refer to the combustion of by products of the primary combustion, which is something of a fiction in this case, for the white inner flame IS actually is the primary flame envelope in this case, and the blue flame is the secondary flame envelope here, so that what is normally considered as the secondary flame envelope is actually the third envelope. How to resolve this; don't go there buy or build a good enough burner to see now white in the flame, and then tune it up well enough to have none.

The next question tends to be "how dark a blue?" Different fuels give off different hues, and lean flames are always darker blue than neutral flames in any given fuel. In fact I made one burner that could be run so lean that the flame turned purple from the amount of red that could be included in it. On the other hand, any slightest tinge of green in the flame is an unmistakable sign that it is fuel rich; such a flame will be pumping out dangerous amounts of carbon monoxide.

You can also get thin yellow and red streaks in a perfectly tuned burner's flame, due to breakdown products of oxidation from some alloys of stainless steel. Flame nozzles #304 stainless can put on quite a show that way; it's harmless.

Normally, methane gives off the darkest blue of all available fuel gases, yet a compressed air/natural gas torch I used at one school for several months put out the lightest blue I've ever seen in a high speed air/gas flame; this was probably due to total primary flame combustion; something I highly recommend with other fuel gases, but not from natural gas.

Air/butane flames from so called "blue flame" torches are darker blue than from air/propane flames, yet butane pocket lighters started out being set up to make soft yellow flames.

 

But isn't the exhaust flame just the tale end of the burner's flame alter all? Yes, it can be just that in a forge that is loping along, but in a forge turned up into yellow to white heat ranges...NO. In fact the goal is no output flame at all; just clear super-heated flue gases. If you have a forge and burner capable of this kind of performance everything else about the exhaust changes too.

With the average forge a small amount of blue exhaust flame is considered normal, but in our example of a really hot forge, if you keep turning up the input flame beyond the forge's ability to completely burn it, you still won't get blue exhaust flames; it will spew yellow white exhaust flames that will complete combustion within a few short inches, instead of massive amounts of carbon monoxide.

What changed? The forge itself is changing the combustion equation  by super-heating the byproducts of the primary combustion envelope before it comes into contact with secondary air.

How is this possible, since immediately after combustion, flame temperatures naturally decline? Radiant energy input from the incandescent forge surfaces is being bounced back and forth through the by products.

If the forge is orange hot you could consider heat losses in the byproducts to be multiplying faster than radiant energy is being added. In a white hot forge losses are being subtracted while radiant energy is multiplying their gains. It isn't possible to understand internal combustion processes in a modern forge as just a chemical process, because heat gain from radiant surfaces are higher than that from primary flame combustion.