[TheForge] Re: Castable refractory

[Tom Troszak]

At 11:38 PM 5/15/2003 -0500, you wrote:
> >larbooks@impulse.net wrote:
>Now I want to make a forge that is lined with Mizzou castable refractory
>3000 degree.Just got the refractory today (55 lbs for $27.50)  but no info
>with it or on the bag as far as specifications or applications are
>concerned.  I assume I just mix water with it and not make it too
>soupy--I'll just wait plenty long for it to set up.  What I'm wondering is:


Dear Norm, I have built many forges and industrial furnaces (more than 100) 
using different kinds of castable refractory: here are several things for 
you to consider based on what I know from my experience:

The denser the refractory, the more conductive of heat it is, therefore the 
furnace will will require more heat input to maintain temperature.

The 3000 deg hard stuff you bought would be perfect for burner nozzles or 
places where the fire impinges directly, but it will cost you dearly in fuel 
if you use it for the whole lining, and you may not be able to reach welding 
heat.  Once fired the hard castables will eventually attain the hardness of 
glass or pottery, and be a real pain to remove.

There are other castables with the word "insulating" in the title, and can 
be had up to at least 2800 deg F. rating, and these would be a better choice 
for the entire lining and roof  of a furnace, save you fuel, and make it 
possible to reach welding temp.

Be very careful about the water when mixing, a little too much is way too 
much. If I recall correctly, the ratio works out to about a 1 to 3 ratio, 
that is to make 40 lbs of cement use 30 lbs of cement and 10 lbs of water. 
If you are unsure, contact me off list and I will dig up my exact formulas.  
Weigh out the materials in seperate containers, and add about two thirds of 
the water and mix vigorously untill every granule of the cement is wetted, 
if necessary, add the remaning water just a splash at a time, then remix the 
entire batch until right.  You can buy little stainless steel needles from 
the cement guys, and mix about a double handful into the dry mix for each 
batch, about 1 or 2 lbs per bag, and these will work like straw in bricks 
and make the whole affair much tougher when finished.  Careful, they are 
kind of pointy, but worth it.

Important: You must mix VERY QUICKLY, and very thoroughly as the cement will 
begin to harden in about  20 minutes whether you like it or not.  Have your 
froms ready and LEVEL before you start mixing.

You can mix the cement very dry, like putty, and apply it to the form by 
hand, or mix it a little wetter, like very thick outmeal, and scoop it into 
the form and tap the form to make it settle.  It should be thick enough to 
stay heaped when dumped from the trowel, but flow slowly into the form when 
the form is tapped.  I use a needle scaler or auto body hammer to "buzz" the 
form and get the cement to flow, or you can just tap it by hand.

NOTE: The standard method of mixing cement for the garden walkway, i.e.,  
just dump the powder into a trough and add water with the hose until "it 
looks right" will absolutely DESTROY the quality of the finished product, 
the cement will crack while setting, and fracture even more when heated.

O.K. here is the overall idea:

The various fiber wool, fiber board, are the most insulating and efficient 
of fuel, welding temp is easy to attain, but ther are very fragile.  They 
are easy to replace. these are the lightest materials, if portability is an 
issue. Furnace will heat veru quickly, (3-5 min) and cool very quickly.

The white high alumina bricks -"very soft, light, fluffy bricks"- are next, 
very efficient, but very fragile, and they must be supported because after 
they are fired once or twice they will fall apart upon handling. They are 
very light, and work very well if you don't ram cold steel into them too 
much.

The "insulating castables" are MUCH more durable, but slightly less 
efficient insulators, and are a good choice for all around furnace 
construction, welding heat is still easily attainable, but uses more fuel. 
Make walls at least 2-1/2" thick. furnace will heat more slowly (5-15 min) 
and take longer to cool.

The "hard castables" are extremely tough and perfect for burner nozzles, or 
high traffic areas, but extremely heavy, and very difficult to remove. make 
walls at least 4" thick, or you may never get the furnace hot, as the heat 
just leaks out too quickly.  welding temp may not be attainable with Nat. 
gas unless walls are very thick.  Furnace will take forever to get hot, and 
even longer too cool off. Extremely heavy.  Not recommended for floors. 
Excellent choice where fuel is free, and there is severe abrasion happening.

Speaking of floors, remember that all refractories expand and contract with 
heating and cooling every time the furnace is fired, and all will generally 
swell up over time, so leave room in the design for the refractory to 
"breath a little".  If you pack cement into a solid metal box, it may 
fracture the first time it is fired.

For furnace floors, I like to use hard 3000 deg F. firebricks very loosely 
spaced (1/4" MINIMUM) between bricks, use a piece of fiber board under the 
floor bricks for max heat retention, or fill gaps with sand. The bricks will 
slag away and swell noticebly over time, so unless you own a jackhammer, 
leave a generous space when placing them.

I let the cement cure at room temp (below 60 F. it may not set for days) and 
if you have done everything right, the body of the casting will begin to 
stiffen noticably and become sort of "granular" within 30 minutes, and 
become warm to the touch within a couple of hours. I leave my castings 
absolutely still for at least 8 hours, overnight is best.

After the cement has set (will resist a thumbnail), remove the forms and 
cure. I use a kerosene heater to "bake" the cement (200-300 deg F. max) 
overnight  to drive out moisture before it is fired. The cement is now fully 
cured, but there it still enough water in the matrix to crack the casting if 
you heat it up too fast.  The poor man can just fire until steam appears 
(but before redness) and let cool. repeat until you can't stand it any 
longer, then fire that sucker up.

Oh, yes, wall and roof anchors are a great thing, I use scrap bolts and 
cament nails to stud the inner walls of doors and things like that, so with 
stud anchors, and the stainless needles, your lining will not fall out even 
if it DOES fracture in a hundred places.

Forms can be anything wood, metal, whatever, stiffer is better, if the form 
bulges when you fill it up, your casting will have a bulge in it.  Forms do 
not have to be waterpoof, but gaps larger than 1/16 will ooze cement. I use 
vaseline for a release on my forms, and when strangers in the shop see a big 
tub of vaseline sitting out and ask what it is for, I just raise one eybrow 
suggestively, and wink...

If you have other questions, please ask.

I hope this is helpful, your experience may vary, no salesman will call...
Tom Troszak

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