Forges 101

[BayardStrachan]

Hello all, not sure if this is the right place to post but here goes. 

I am wanting to build a small gas forge and I am thinking if using bricks from a demolished fire place.

First question is: Would they be rated for the temperatures in a forge?

Second question: kinda random but I have seen several tutorials online that say plaster of Paris and kitty liter makes a good refractory cement. Sounds like B.S to me but what do you think?

Thanks in advance 

[swedefiddle]

Good Morning Bayard,

Welcome from the left Coast of Canada.

The short answer to your question, is Yes. I have built Forges with 9 bricks a few times. The burner can enter from the side, bottom or from the back. This type of Forge is not the BEST Design, but it is very functional. it will take 15-20 minutes for the bricks to come up to good heat, Yes there will be a lot of Heat wasted. You will not be welding in a 9 brick Forge, but you will be Forging. The key is, do something and try it. Change it a little and try it again, continue until you are satisfied. Enjoy the Journey, keep track of your creations in a scribbler, what worked. What worked better, etc.....

Neil

[ThomasPowers]

Hard Firebricks: You will spend more on gas in a very short time than you would have spent on a better refractory, it will take long to heat up and long to cool down to safe temperatures. 

As for Plaster of Paris and stuff;  plaster of paris starts to degrade about 1000 degF below good forging temps.  It's a classic example of how BAD information gets spread on the internet---and a good touchstone; I'd not trust anything a site that recommends it has on it without a lot of double checking.

[BayardStrachan]

Yes ok thanks Thomas and swede fiddle, I want to be able to make damascus in it so I need to get it to welding temp. What about putting ceramic Fibre behind the bricks, would that do anything or a waste of money and time?

The bricks were free so probably going to give it a go by themselves anyway.

[Frosty]

Fire place bricks are NOT rated high enough for forging, it will degrade quickly as in the first time it comes to forging temps. 

If you're going to build a brick pile forge use suitable insulating light firebrick. Morgan Thermal Ceramics, K-26 insulating fire brick are rated to 2,600f which is hot enough to forge weld. IF you do it right.

Wrapping the outside of a heavy brick forge in insulation will help keep the room cooler but won't make a heavy brick forge much better than none.

How much of "Forges 101" have you read? There are many discussions about forges, pros, cons, mods, etc. it's well worth skimming and reading what interests you. There are quite a few proven forge designs to copy.

Frosty The Lucky.

[BayardStrachan]

Thanks frosty for the advice, have read about half of forges 101. Decided that I might build one from completely new supplies, doesn't sound quite as complicated as I originally thought.

[BayardStrachan]

What about this forge design, looks very simple. Becoming slightly skeptical of things on YouTube so looking for some different opinions.

youtube.com/watch?v=ZNqYG5_f7so

 

 

[JHCC]

That’s basically just a steel box lined with fire brick (hard for the floor, soft (insulated) for the sides and top). Simple to build, but it won’t hold in the heat as well as or be as durable as one made with ceramic wool and castable refractory.

[Mikey98118]

10 hours ago, BayardStrachan said:

First question is: Would they be rated for the temperatures in a forge?

NO!

[Frosty]

I guess that's one way to make a brick forge but leave out the hard firebrick floor and use something else like kiln shelf over Kaowool or similar. If you built it with Morgan K-26 fire brick you could kiln wash the interior and the floor is good to go. 

Rather than going to the trouble and expense, unless you have the welding equipment and way to cut the steel for a steel box, consider the one below. It has about the same volume and dimensions as the one in the video but doesn't require the shop tools or equipment. The burner Alec uses is an excellent burner but way more than should be necessary for that volume forge.

Our club built more than 30 of these in a two day burner and forge clinic. We purchased all the materials in bulk so building a one off forge will be more expensive. The club added IIRC 15% to grow the club coffers so they cost members right at $100 ea. Non members paid another $35 IIRC. 

If you count bricks you'll see it's the same # and arrangement as in one the video. The only tools you need is a hand drill, hack saw and tape measure. To build the 1/2" T burner a drill press is a BIG advantage. My design T burner is found pinned in the gas burner section here.

This forge had been running maybe 4-5 minutes when I took the pic an enthusiastic member put the steel in as soon as it was lit. Once the forge is heated up steel that size comes to that temp in a couple minutes. One of the guys did a forge weld in one of the others during this meeting and one of the new guys got to talking and melted the 1/2" sq. he was heating.

There are professional bladesmiths in the club who have been using these exclusively except for large work since building one. 

What doesn't show is all the firebrick was kiln washed with Plistex before assembly. We dipped the bricks in a bucket of water with about 1/2 tsp Elmer's white glue mixed in then brushed with Plistex mixed to thin latex paint consistency. We kiln washed the brick the first day before the burner build clinic got started so it was dry before the forges were assembled. Yes, the burner ports were drilled before they were kiln washed.

This little forge is in line with my reasoning for developing the T burner, it's effective and requires minimum shop equipment and skills so it's available to almost anybody who has or has access to a drill press and vise. With a little thinking and measuring you can change the size easily.

Frosty The Lucky.

[BayardStrachan]

ok thanks frosty once again. Small issue, here in NZ i cannot find 1/2 inch iron pipe anywhere smallest i can get is 15 mm, i have some 25 mm and fittings on hand, but am i right in supposing that a 1 inch burner in a small forge like this is just excessive?

also i cannot get morgan k26 bricks, i have found these though, at least they have a 26 in the name

[Mikey98118]

I have been playing around with metric tubing, and some metric pipe. Your nominal pipe doesn't seem to be as oversize as ours is. I would guess that what you have is close enough to 1/2" pipe.

The photo shows the same rough surface as standard semi-insulating firebricks, that loads of guys are using to build forges; chances are they will do okay.

[Frosty]

JM26 are Morgan Thermal Ceramics IFBs (Insulating Fire Brick) and according to the Morgan site equivalent to the K series of IFBs. I have no experience with JM series but the analysis in the photo is high alumina and the rated temp looks good. The details list gas fired applications. 

Even at $175 to build a forge like our club builds, propane at $4/gal usd. would reach payback in a few day long sessions compared to a heavy hard firebrick forge.

I have made and used a forge built from heavy fire brick and it would go through a 5gal. tank of propane in under 6 hrs and required a 3/4" burner to reach working heat in under half an hour. The brick pile forges as shown use a 1/2" (15mm is close enough) come to working heat in about 5 minutes and 5gal. propane at approx $25usd. lasts about 16 hours. Hard firebrick are approx $5 usd each here. 

I built my first Kaowool outer liner with hard castable inner liner forge after that first brick pile and while it was a long way from economical it was 4x as economical. 

I think I've decided to stop recommending ceramic blanket refractory since seeing the information about Morgan Thermal Ceramics, "Superwool EXTRA". I may have to bug our local HVAC supplier if they haven't started carrying it already but the stuff is NOT the breathing hazard ceramic fiber refractories are. The performance looks as good, maybe better than 8lb. ceramic blanket.

I know that doesn't apply to your question but it's important information, health and safety wise.

Frosty The Lucky.

 

[Mikey98118]

I agree. Those Morgan bricks trump ceramic fiber blanket, and end up costing about the same. Just don't forget to use a finish flame coating on their inner sides.

[Mikey98118]

Understanding flames and atmospheres inside forges: When looking at the flame from a really hot burner in a cold forge or furnace, It will appear much as it does out in the open air, but within moments it will lengthen and become smoother in outline, as the equipment starts to super-heat; it will also lighten in hue, becoming more transparent. There will be little to no secondary flame within the equipment, even while it is cold; lesser burners will make more complicated flame envelopes, but this is the ideal; these facts also hold as true for multi-flame ceramic burner heads as they do for single flame burners.

    You need to remember that there are at least two different flames going on within the average gas forge or furnace; the flame being input by the burner, and an internal atmosphere, which may extend to an output flame leaving the equipment via the exhaust opening. When blacksmiths discuss terms like dragon's breath it is such an exhaust flame they are speaking of; a very different animal than the burner flame. Not that both flames aren't equally important clues to burner performance, but they need to be treated separately for clarity. So, what amounts to a perfect exhaust flame? No flame at all.

    If we are speaking about the burner flame, straight blue from a single combustion envelope is the goal, 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 blue or purple tertiary flame; this flame comes from combustion of byproduct gases with flame induced air through a burner port. Buy or build a good enough burner to see no white in the flame, and then tune it well enough to have little or no secondary flame. Then control flame induced air with a secondary choke on your burner’s mixing tube.

    The next question tends to be "how dark a blue?" Different fuels give off different hues, and lean flames are always a darker blue than neutral flames in any given fuel. In fact, a burner can be run so lean that the primary flame turns purple from the amount of red that excess super-heated oxygen gives it. On the other hand, any slightest tinge of green in the flame is an unmistakable sign that it is way too fuel rich; such a reducing flame will also be pumping out lots of carbon monoxide.

    The simplest way to judge a neutral flame is that it’s blue is a lighter hue, and it has very little to no secondary flame; any darkening beyond that is from too much oxygen; it is called a lean flame as it is thought to be lean on fuel as compared to air input. In the end, you must tune a burner back and forth between rich and lean to educate yourself on what constitutes the best flame from your burner; you can do this out in the open air, or in the equipment, while it is warming up.

    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, mild steel, or cast iron in flame retention nozzles. Flame nozzles of #304 stainless can put on quite a show that way; it's harmless. #316 stainless nozzles make fewer streaks and last longer.

Fuel rich (AKA reducing) flames, range from the faintest tinge of green in a blue primary flame envelope (AKA flame front) to bluish green flames that are pushing so much un-burned fuel into your shop's atmosphere that you feel like gagging. If the burner’s choke is completely closed the burner will make a lazy yellow flame like burning wood.

Neutral flames range from light to medium blue; they are neutral throughout this tint range for all practical purposes; what that means is, although their combustion chemistry is changing, you can't appreciate the difference without calibrated instruments.

    So how can you know when the blue leads the neutral range and inters oxidizing? The answer is that you can’t without a fair amount of practice. Eventually, you will learn to compare the flames from your burner at one time and another, so as to tune it perfectly.

Oxidizing (AKA lean) flames start just beyond medium blue, go through dark blue, and extend into purple, in the primary flame envelope. While learning to discern the boundary between neutral and oxidizing flames, it is helpful to use small pieces of fresh ground steel in the forge, how fast and how much it scales—in the forge—gives you a faithful comparison, as you self-educate about flames. 

    Flame color isn't the only sign of how well your burner is doing. The amount of secondary flame is also an important indicator; the less secondary flame the better. There is such a thing as perfect performance, which includes no secondary flame. Perfection is often the enemy of practicality. A small wisp of secondary flame is often better than no secondary flame at all; this is because air/fuel flames fluctuate more than oxy-fuel flames, so the "perfect" flame is likely to be slightly oxidizing part of the time. Since a wisp of secondary flame will burn up completely in the forge or furnace, it is better than scale added on work pieces during heating, or oxidative damage to super-heated crucibles. It should go without saying that tertiary flames indicate poor burner construction, or a very bad job of tuning.

    So what is the practical upper limit for secondary flame? Is there flame coming out of the exhaust opening? Then your burner is either tuned to rich, or its gas pressure is turned up to high.

    Even with the best possible flame (that you can detect visually), there will be some superheated oxygen molecules that haven't had time to combine with fuel gas molecules before escaping the primary flame envelope; not enough that you can see secondary combustion going on. But, any superheated oxygen that impinges on super-heated metal, will joyfully combine to rapidly create scale, and burn away some carbon content in ferrous metals. What this means is that a few inches distance between the visual end of the flame and your work pieces, or crucible, is highly desirable. Hot crucibles are inclined to suffer damage in the presence of super-heated oxygen, leading to spalling, cracking, and early crucible failure.

    It is an advantage to build a tunnel, oval, or “D” forge with the flame angled away from heating stocks (and between the crucible and wall in furnaces), or with the ceiling at least far enough from the work in box shaped equipment, to keep the flame from impinging on heating stocks and crucibles; increased room for the flame is one of the reasons for including a plinth in your casting furnace. Since different burner designs create different flame lengths, and since they also vary by how far the burner is turned up, there can be no pat answer on the height of a box forge or the thickness of a plinth in your casting furnace; these are judgement calls on the builder's part.

    Most people find little reason to turn a burner on full blast, so the flame can be measured for length at a maximum of 20 PSI, and that can be used for a good height measurement in box forges. You want the length to be at two-inches beyond secondary flame tips. No practical forge can include further length for tertiary flames, so construct and tune your burner well enough to avoid making them. Crucibles are tapered at their bottoms and should be raised on plinths to help keep the flame from impinging on them, since most casting furnaces are round and have a burner placed low on its vertical wall, and aimed horizontally at a tangent between furnace wall and crucible walls.

    However, flame length is most important if your burners are top mounted and facing toward the equipment’s floor. Some people mount their burners high up on sidewalls facing horizontally across a box forge or furnace, to get around early flame impingement altogether; this saves heating stock, and also lowers thermal damage on walls (which can be further away the work, for the same cubic inches when ceilings are lowered; a win-win use of space).

    Is an exhaust flame just the tail end of the burner's flame? It can be just that in equipment that is loping along, with interior surfaces that are only at red or orange heat. But in a forge or furnace that is turned up into yellow to white heat ranges—no. In fact, the goal is zero output flame; just clear super-heated exhaust gases. When your forge or furnace is capable of radiant-oven performance, then everything about the exhaust discharge changes.

    With the average forge or furnace, a small amount of blue exhaust flame has been considered normal--in the past. But in really hot equipment, should you keep turning up the input flame beyond its ability to completely burn internally, you still won't get blue exhaust flames; some of the yellow-white “atmosphere” will overflow out of the exhaust, and complete combustion within a few short inches, but without a trace of blue or purple flame (which indicates a probable buildup of carbon monoxide).

    What is different? The forge or furnace itself is changing the combustion equation by super-heating any byproducts of the primary combustion envelope. How is this possible, since immediately after combustion, exhaust gas temperatures naturally decline? Intense radiant energy from incandescent surfaces is being bounced back and forth through those gases. All of the interior becomes an ignition point; not just refractory surfaces. Thus, secondary combustion will be increased, ensuring left over products of the primary flame envelope have the time to completely burn off.

    If the equipment is red-hot, you should consider heat losses in combustion byproducts to be multiplying faster than radiant energy is being added. In pale-yellow to white-hot forges, combustion losses are being subtracted while radiant energy is multiplying temperature gains. It isn't possible to understand internal combustion processes in a modern forge or furnace as just a chemical process, because of heat gain from highly radiating surfaces; such equipment is as much radiant oven as flame appliance.

     So, exhaust flames from your forge can simply be the result of fuel that hasn't combusted, as in the case of fuel gas pressure being turned up way too high. The more common cause of yellow exhaust flames is a large secondary flame (from a poorly designed, constructed, or tuned burner), which cannot be completely combusted within the forge interior.

    I have noticed that fairly opaque looking yellow to orange flame can be made from some kinds refractory that is "cooking off" calcium from its binding agent; these flames will not abate until the process is complete. As the flame turns from yellow to orange, it becomes more transparent, and may even seem to sparkle in a manner reminiscent of fireworks, if the forge is running hot enough at the time.

    This doesn't preclude other colored flames, such as purple and blue from being present in the orange exhaust, but they are an indication of poor combustion, and must be ignored until the refractory finishes out-gassing. It is best to address one problem at a time.

 

Caution: Blue exhaust flames are a sign of a reducing forge atmosphere, which even a perfect burner will give off, if its air intakes are choked enough. Be aware that blue exhaust flames will be accompanied by carbon monoxide production. Carbon monoxide monitors are cheap and effective health insurance.

 

[Mikey98118]

 

Equipment burner sizes

Generally speaking, burner sizes are based on schedule #40 pipe sizes; or rather on their nominal inside diameter (or its rough equivalent in tubing). Classic venturi burners (AKA wasp-waist, such as Ransom burners) go by the throat diameter (the narrowest point of their venturi constriction).

    One naturally aspirated 3/4” burner (that is capable of making a neutral flame) will heat 350 cubic inches of interior volume to welding temperature in a properly insulated forge (two 1” thick layers of ceramic fiber or the equivalent insulation in some other form). Add additional cubic inches for a burner capable of making a neutral flame in a single flame envelope (which can be tuned from no more than a trace of secondary flame into an oxidizing flame). Below is a list showing how this will translate in other burner sizes. If you substitute schedule ten stainless steel pipe for schedule forty mild steel, you can add a little more heat in forges and furnaces.    Controlling secondary air being induced into the forge through the burner opening(s), by the burner flame can raise efficiency up to twenty percent more.

(1) A 1/4” burner should sufficiently heat a 44 cubic inch interior from a bean can or two-brick forge to welding heat. If you just want to shape parts or heat treat them, the burner will do for a 60 cubic inch interior.  

(2) A 3/8” or two 1/4” burners can adequately heat 88 cubic inches; enough to weld steel, or melt bronze, in a two-gallon cylinder forge or casting furnace made from a non-refillable Freon or helium cylinder, or a mini oval forge made from half a car muffler.

(3) A 1/2” or two 3/8” burners should adequately heat 175 cubic inches; enough to run one of a small brick-pile or box forge, or two gallon cylinder tunnel forge.

(4) A 3/4” or two 1/2” burners should heat 350 cubic inches; enough to run a refillable five-gallon propane cylinder forge (or the equivalent size casting furnace).

(5) A 1” or two 3/4” burners should heat a 700 cubic inch interior for a small pottery kiln, etc.

(6) A 1-1/4” or two 1” burners should heat a 1,400 cubic inch kiln.

Use of a ¾” burner with a perfect flame (total combustion in a single envelope), which is mounted in an entrance port that is set up to control how much secondary air is induced into the forge by that flame, and you can add another 14% to the volumes listed above. Addition of the proper heat reflection coating will raise forge or furnace temperature still further or reduce fuel used to gain yellow heat; putting it another way an optimal burner running in an optimal forge or furnace will do the same work as average equipment with about 30% less fuel.

    So why not use cubic volumes to describe every burner size? Actual numbers will vary according to burner and equipment designs. On the other hand, naturally aspirated burners all have very long turn-down ranges. If you are anxious about using a hot enough burner for your forge or furnace, use the next larger burner size, and turn it down. Later on, if you long to get the burner size just right, it’s easy to change your burner out for a smaller one, but the reverse is not true.

[BayardStrachan]

i am looking at a burner that advertises 41 mj/h, what size forge body would i need to get to welding temps?

also, i have a question about frostys mini brick forge, would it be at all limiting? like i want to be able to spend money and be able to use it until it is stuffed without having to not do things becasue it was too small

[Mikey98118]

The type of forge you are asking about is what Frosty likes to describes as a "brick pile" to emphasize what he most likes about them; that, they are endlessly changeable and enlargeable.

19 hours ago, BayardStrachan said:

i am looking at a burner that advertises 41 mj/h, what size forge body would i need to get to welding temps?

I haven't paid attention to BTU per hour claims for burners in twenty years; they make a nice advertising gimmick, but have very little to do with choosing the right burner for any given forge, or visa versa. In the first place, all burners are not created equal; so how much fuel they can consume per hour only has a shaky connection to what how large a forge they can heat to yellow incandescence, which is where you want your forge for welding.

[Frosty]

I'm having trouble taking your question seriously. Did you read what you asked before hitting submit? 

Of course the size is limited, NO container can hold more than will fit.

Try thinking about asking a good question and try again. I'm happy to help but . . . 

Frosty The Lucky.

[Mikey98118]

Just forget about using pipe as your mixing tube, and look for round tub in a roughly equivalent inside diameter.

[BayardStrachan]

ok thanks for the replies frosty and mikey. sorry if my question didnt make any sense frosty, i think mikey answered my question well though. 

[Mikey98118]

All that matters about pipe sizes is there inside diameter, because everything on this site is figured by those inside diameters. You will find round tube close to, are exactly the same diameters. When you can't find a tubing match to the exact pipe I.D., try to go a little larger; not smaller.

[BayardStrachan]

Is kaowool with a temp rating of 1200 degrees Celsius ok for a forge. Just that it would be cheaper to make a kaowool forge with a  refractory cement flame face than to do a brick pile forge. If I can use 1200 degree kaowool. I am just on a tight budget and I am trying to get the best value 

[Mikey98118]

That would end up being the same extended use temperature rating as 2300 F Kaowool, which is pretty standard in oval, tunnel, and "D" shaped gas forges. Unless you use 2600 F ceramic fiber insulation as your secondary insulation, you must consider the refractory blanket as a consumable.

How long it lasts depends on how hot you heat your forge, what you use as a flame face/primary insulating layer, and whether or not you rigidize the blanket. I would also recomend a heat reflecting finish coat, if you can find something suitable.

This is not meant to discourage you. All these fatcors are under your control; they just call for value judgements as to how far you want to much yourself out, to do how good a job, in return for adding years before your forge needs to be rebuilt.

[Mikey98118]

Employing a propane adapter hose

 

Propane extension hoses (AKA adapter hoses) are marketed to allow campers to run portable gas stoves on refillable propane cylinders, instead of the 16 oz. none-refillable canisters that backpackers prefer; some of them also allow you to do the same with canister mount torch-heads on burners.

    These hoses all have a QCC type connector fitting at one end (which screws into a canister-mount burner design). The hose’s other end may have a QCC connector fitting meant to mount on a canister, or a POL fitting for refillable cylinders. Even the hoses with QCC connectors on both ends are helpful, as they allow the burner to operate as a hand torch more easily, and to separate the fuel source from hot equipment by a few feet.  

    There are hoses that include variable pressure regulators, and hoses of varying lengths. Hoses with stainless steel armor braiding are the best choice, whatever their length, for service near hot equipment (or in busy metal working shops). I don’t like standard heavy wall propane appliance hoses sold in stores; they are stiff and overpriced. However, adapter hoses aren’t stiff and have reasonable prices.  Braided armor stainless steel propane hoses, are also available with 3/8” flare nut connector ends, which can be matched up to propane flare fittings available at some large hardware and appliance stores.