Forges 101

[Mikey98118]

 

Making an end-run around recomended forge sizes

It is a natural desire to go for the largest forge you envision yourself ever needing; and that is nearly always the wrong move. There is no such thing as a forge that is too small; if you outgrow a smaller forge, you will still find yourself using it whenever possible, to save time and money. On the other hand, an oversize forge usually ends up collecting dust in a corner--to save time and money.

    Building costs of forge construction are directly proportional to size. What’s worse is that you may end up choosing second rate refractory materials, and burners, to save on construction costs in larger forges; this can end up tripling the expense of running your forge. The larger the forge interior the greater the heat loss through the walls, ceiling, and floor. The greater the heat loss the more fuel that must be consumed to keep the interior super-heated, which leads to increased heat lost through the exhaust opening.

 

A five-gallon used forge/furnace is as close as you’ll get to having your cake and eating it too: Most home casters use a five-gallon used propane cylinder for their furnace's shell. I normally suggest a smaller shell for a first gas forge, but if you also want to build a casting furnace, the five-gallon propane cylinder can usually be picked up for free at most places that sell propane; this is because cylinders must be tested every ten years, to see if they can be legally refilled. There are usually old cylinders lying around that can't be refilled; they are happy to give them away.

    The biggest difference between tunnel forges, and casting furnaces are whether they are positioned horizontally, or vertically. So, an extra four legs on the shell's far end (at least 4" length) allows both uses. You also need to have the near end carefully cut off, in a flat plane (hopefully using a welded seam on the cylinder as your guide), and hinged back on, with a latch added on its other side, so that crucibles can be lowered into the furnace and picked up out of the furnace with special tongs. Some people include A hinged door on the exhaust opening end of their forge, for ease of refractory installation and repair; also, for the ability to heat larger parts in the forge than would otherwise be possible). You will also want the inside surface of the far end of that forge to be perfectly flat; I recommend using a small round kiln shelf for the purpose, instead of cast refractory; it's just easier to get right. Instead of a smaller exhaust opening, you drill an emergency spill hole in the far end, in case of crucible failure (at least 3/4" diameter). Instead of a flat forge floor, make the wall entirely round, and slide a rectangular kiln shelf into it, to serve as floor, for forge use; leave it out for furnace use. Use two 1/2" burners (placed a one-third distance between both ends), instead of a single 3/4” burner. You shut down the top burner during casting, and only use it for forge work. You can also place a temporary internal baffle wall on the kiln shelf floor, and shut down the far burner, to save fuel when heating small articles.

[Jimw3326]

On 1/7/2023 at 5:38 PM, ThomasPowers said:

My family leans heavily towards tea; It seems like we visit the St James Tearoom in Albuquerque often enough that we "should call it St Jim".

If it's the one at Edith and Osuna, I pass it twice a day. It's 2 blocks from work and 1 mile from home.

[ThomasPowers]

Yup; that's the one.

[Mikey98118]

Homemade insulating refractory: Homemade refractory isn’t a smart choice, for anyone who can find the kind of refractory desired in their local market; otherwise shipping charges can be more than the cost of the product. The greater the amount of refractory, the higher the shipping fees go. Usually, this want of local refractory is a lack of the insulating variety. So, below are some solutions for this problem:

 

A buddy used regular cast refractory for his casting furnace, mixed half and half with Perlite from the garden department of a large hardware store. His burner was a very hot tube burner design, and though he was only casting aluminum at the time, he always ran his burner maxed-out. In fact, he oxidized two stainless steel flame retention nozzles away in six months! His furnace lining is still in excellent shape.

Here is another guy’s description of how he, and one other, make different homemade refractories. It should be noted that what is sold as “furnace cement” is changing; some of it is, but some is castable refractory, at higher prices than is found in 50-pound refractory bags:

“If you can get your hands on ready-made furnace cement, preferably of the “black” 3000 °F variety used to make repairs on wood-burning stoves, a simpler recipe is provided by John A. Wasser: You will need about 1 part (by volume) of Furnace Cement for each 4 parts (by volume) of Perlite so for a two-gallon bag of Perlite you will need a half gallon of Furnace Cement. If you use much less than four volumes of Perlite for each volume of Furnace Cement all of the passages between Perlite beads will be sealed,

and it will take a long time for the cement to set (it needs contact with air). If you use much more than five volumes of Perlite for each volume of Furnace Cement the resulting material will be quite weak. You will also want to have some Furnace Cement to use as a sealing coat on your lining. The Furnace Cement has about the consistency of roofing tar and is very sticky. It is MUCH easier to work with if you add about 2 cups of water per gallon of cement. This makes it more like a thin plaster.”

 

    You have probably noted that this advice flies in the face of the half and half formula my buddy used. Back in those days our casting groups allowed an entire week for the cast refractory to set up, and this was followed by a very slow increase in firing temperatures, until no out-gassing of steam was observed.

[Mikey98118]

Four different free from-scratch recipes for insulating refractory are available at: https://pacificmolds.com/blogs/casting-journal/refractory-cement

[Mikey98118]

It is all too easy to encourage false impressions, and all our talk about going out of your way to create swirl in your forge atmosphere will do so, if it isn't balanced with a review of their limits, from time to time.

It doesn't take a lot to encourage swirl in even a box forge; just tilt the flame at some angle, rather than dead on to the first surface it will impinge upon, and let nature take its course.

So, while we encourage swirl, and therefore discourage top-dead- center-facing-down burner positioning, as a faux pas, it's not the end of the world. This is even better news to all you cheap- skate cheaters (like me), who take a building shortcut, by buying an inexpensive little oval forge

One of the many little happy details about their shape is that, while they all have top-dead- center-facing-down burner positioning, the forge shape will encourage the flame to swirl away from the work surface in two separate directions; half a loaf is so much better than no bread, don't you think?

[gewoon ik]

question about the re-emision mixture you mentioned a couple of times : 95% Zirkopax and 5% bentonite.

Is it on weight or on volume? 

Both are relative cheap to buy at the ceramics store but in big quantities.

zirkopax : 1kg for 16euro and 250gr for 8 euro

bentonite : 1kg for 4euro

I'm still looking for a flamecover. The one supplied with my forge is on some places worn out (from rubbing against it with hot metal, can you image that ). And I do find proper refractory but smallest pack is 5kg. A bit much for a small repair, i can plug my forge with that quantity.

 

 

Some other info (for EU-persons can be interesting).

I found a cement Electroland (used by potterys for repair of their kilns), rated till 1300°C (2372°F). I put their sheet in attachment (if it's not allowed by forumrules, sorry, delete it).

it is high in Ca0 (37,4%) and Al2O3 (38%)

Can somebody with a bit more knowledge tell me if this can work or not?

 

 

11410-CMI-Fitxa-digital_ELECTROLAND_ENG.pdf

[Mikey98118]

Zircopax Plus (also known as Superpax) has replaced Zircopax, which is no longer available. All the information you are seeking is still available in the original Zircopax article on Digital fire:  https://digitalfire.com/material/zircopax As to your particular question; formulas are normally considered to be by weight, unless “by volume” is specified, which it was not.

4 hours ago, gewoon ik said:

Can somebody with a bit more knowledge tell me if this can work or not?

There are two strikes against it: In the first place refractory cements are only meant to bind refractory bricks together--not used as a flame face. Furthermore, it use rating of 

2372°F is normally used on secondary insulation product; it is far to low for primary flame faces in anything more than a wood stove.

[LeeJustice]

Gewoon, I am personally hesitant to use Calcium Oxide inside my forge.  It does have a really high melting temperature, and, when heated to high temps, emits light.  Hence, the old saying, "being in the limelight".  Why the, would anyone not want to use it?  It sounds good.  A quick internet search quickly reveals:

Short-term Exposure: Calcium oxide causes irritation of the eyes, nose, throat, and skin. Severe burns may result from contact with this chemical. It may also cause bronchitis and pneumonia. Exposure to calcium oxide may cause irritation of the skin and ulceration and perforation of the nasal septum.

[gewoon ik]

Mickey. Thanks. They don't sell zircopax as a brandname, they just sell ZirkoonSilicaat. Could be Zircopax or zircopax plus or could be a different brand. But it is the same thing we talk about, so that is good. 

Lee and Mickey (again )Yeah, some more digging revealt some other alumium cement. Most cement you find are either portland or blastfurnace cement. Both are more or less the same and completely useless for our goal, since they cannot withstand any heat, 500C is about max. So useless and dangerous. The other cementtype is aluminiumcalcium cement. The higher the aluminium, the better against heat I found as a general rule. Amd i found a brand CALTRA, wich has Calight 40, 50 and 70. The numbers are the percentage of Al2O3. So Calight 70 has 70% of Al2O3 and according their website resistant against high temperatures well above 1600C (2912F).

Since it is a cement, it is the base for mortar, concrete, grout, ... 

It comes in bags of 25 kg. So I am now wondering if I can mix it with some isolationstuff and try casting my own bricks with it.

[Mikey98118]

6 hours ago, gewoon ik said:

It comes in bags of 25 kg. So I am now wondering if I can mix it with some isolationstuff and try casting my own bricks with it.

Refractory cement, traditionally, is just another term for brick mortar. However, when it comes to products being sold, there is no such thing as hard and fast rules anymore. 70% alumina content is very likely to be a castable refractory. The primary difference between cement and refractory is solid particulates, which are meant to stabilize the refractory mixture. Other things, like bubble alumina and bubble silica can serve similar purposes; look for them being sold as concrete additives.

[gewoon ik]

it is easier to find at artsupplies than building shops. it is marked as a very good cement to cast and sculpt. But hard on the moulds. 

So Al and Si as additives, thanks. Heat resistant foam stuff in simple terms.

Will see if I have time to dive into that one.

[Mikey98118]

5 hours ago, gewoon ik said:

So Al and Si as additives, thanks. Heat resistant foam stuff in simple terms.

NO! Bubble alumina and bubble silica are very small spheres; when made from silica, they dissolve into the refractory during firing (leaving  spherical voids). The alumina spheres remain as part of the refractory (leaving voids). The result is nearly the same, but bubble silica costs less. While they are a popular method of lightening concrete, and for reducing heat through conduction in refractory structures, they also help to stabilize refractory against cracking.

Well isn't that much like foam? Yup; however, cheap insulating bricks were made by including a foaming agent into clay bricks. We need to keep any confusion between these products from starting.

[Mikey98118]

Of course, you get about the same results using Perlite from a garden supply as you well with silica bubbles, for a lot less hassle and money

The next question will probably be how much Perlite. One-third Perlite by volume is the safe choice. A friend of mine got away with one-half by volume. Either way, I would advise using a flame face layer of pure refractory 1/2" thick on the inner surface.

[Mikey98118]

How will this compare to Kast-O-30 insulating refractory? As to heat conduction, quite well. As to strength; that will depend entirely on the castable refractory used for your base ingredient. I wrote on casting forums for years, where guys used Perlite this way, and before that, they used sawdust, with more limited success. We all switched over to Kast-O-30, not for better insulating qualities, but because of its outstanding record of crack resistance.

[gewoon ik]

Thanks.

I get an answer from remidw about his substitute for kast-o-lit. So thanks. When it heats up a bit, i might buy and try it out.

I i do, i let the results know

[Meep]

Hi gewoon ik what is the substitute if i may ask?

[Mikey98118]

On 3/17/2023 at 9:38 PM, Mikey98118 said:

Of course, you get about the same results using Perlite from a garden supply as you well with silica bubbles, for a lot less hassle and money

alumina bubbles cost more than silica bubbles and a whole lot more than Perlite, so what is their advantage? Remember that both silica and alumina bubbles were designed to lighten concrete, so as to strengthen concrete structures. Both silica bubbles and Perlite dissolve into the heating refractory, just leaving voids. Alumina bubbles remain to strengthen the refractory. On the other hand, this is "gilding the Lilly," so why bother? Know reason comes to mind.

[Frosty]

Lee: The dangerous irritant you're describing is a form of calcium oxide that forms from burning lime, commonly known as quick lime or lye. It's a very dangerous chemical to breath or come in direct un-protected contact with under any circumstances.

The "Calcites" used as binder in modern refractories is almost completely oxdized. It "cements" the refractory by hydrating, not drying. Combustion products from the extreme heat developed in a propane forge produce almost no quick lime or lye type compounds. The bright orange dragon's breath is mostly loose HOT particles and not a combustion product. 

If it were a combustion product it would cease being produced once it was all oxidized. It remains though reduced demonstrating it is not a combustion bye-product. 

Though I haven't lit my forge in a long time it was still producing orange dragon's breath with 4 years of regular use. If the orange flame was calcium oxide it would've driven me out of my shop when I first fired it, instead I put it right to work, I work with bare arms and have very sensitive skin. I can not work concrete without developing a serious rash.

Frosty The Lucky.

[Mikey98118]

Good stuff, Frosty. I think we are going to be circling around the subject of these fumes for some years to come. I have wanted to delve deeper into the topic of refractories; alas, every text I have come across costs hundreds of dollars

[gewoon ik]

18 hours ago, Meep said:

Hi gewoon ik what is the substitute if i may ask?

Arelcrete

For first hand experience ask remidw. He seems happy about it. Don't have it yet, nor tried it

[ThomasPowers]

Out here; Perlite comes by the mountain! They mine it just west of town....

[Mikey98118]

How you doing, Thomas?

[ThomasPowers]

Spending a lot of time in the hospital bed in the living room sleeping.  However only 1 day so far that I had to use the pain meds in the last two months.  Can't drive or go to the shop on my own; but my forge buddies are helping me get to NMABA meetings and providing "watchers" so I can go out to the shop and sort and sell.

Been having a lot of visits from my church vicars; including one from 200 miles away. Worried that I may catch pneumonia from the breeze from the angel's wings---the higher in the hierarchy ones are *spooky*---why the first thing they say when talking with humans is generally "FEAR NOT!"  

[Frosty]

Keep em all hopping to keep up Thomas, the angels will be fine.  I don't know about the vicars but the pastors kept visiting me after the great white incident. 

Frosty The Lucky.