[TheForge] Kaowool vs Soft Fire Brick

Thu Aug 18 20:22:48 EDT 2016

Daniel: First question is a NO, ceramic refractory blanket is a much better
insulator than soft fire brick but it will dissolve in contact with hot
borax like cotton candy and hot water, soft fire brick dissolves like a
sugar cube. They're silica based so have little or no resistance to a
caustic.

I'm not a huge fan of cylindrical forges but they work well on several
levels. Use 1" 8lb. ceramic refractory blanket for the outer liner or
backer. Using two layers of 1" works better than one of 2" for a couple
reasons. First no matter what you do a forge's liner is a wear item and it's
MUCH easier to remove and replace one 1" layer than one 2" layer. 1" blanket
rolls more smoothly so the interior surface of your chamber, (the Flame
Face) will be smoother and cause less disruptive turbulence.

I get 1" 8lb, Kaowool ceramic refractory blanket from a local HVAC supply
and service company, "E.J. Bartell" by name. If you go to a service company
they usually have drops going in the dumpster. They can NOT by law use
scraps in someone's furnace, it must come off the roll. This means a little
nice talk and schmoozing put me in Kaowool to last a life time. Just because
I'm responsible for them selling mucho fire brick and now Kast-O-Lite
refractory doesn't hurt. Our club is on their commercial acct. list. ;)

Cut the blanket a little wider than will roll to size and compress it, it'll
hold itself in position. Use a rigidizer on it, use a colloidal silica or
sodium silicate rigidizer. Do NOT use one of the plastic rigidizers sold for
sealing tile, stone, brick, etc. it will NOT survive in a forge. Wet the
blanket then either spray or brush the rigidizer on and let it set up.

A silica rigidizer does a number of beneficial things in the forge: First it
encapsulates the ceramic fibers so they will not get loose and end up in
your lungs. Breathing sharp particles is NOT good for you, even if it is
pretty inert it's still a bad thing. There are ceramic blanket refractories
that dissolve in water and don't constitute a breathing hazard but I'm not
familiar with them. 

Another good thing about rigidizing blanket refractories, it makes them
stronger they won't compress when you touch them. This will help the flame
face survive mechanical abuse, if what it's laid on flexes it tends to break
up.

Okay that's the backer layer. Once rigidized you can use a sharp knife or a
hole saw to cut your burner ports. More about aligning burners later.

The Flame Face on any furnace takes the worst abuse and propane is one of
the most chemically active fuels going. The three main things that wear out
the flame face are: Thermal cycling, thermal erosion and mechanical erosion.
Silica ceramics are not suitable for furnace liners in that they are
susceptible to both high temp chemistry and caustics. Read the bag if you
buy a castable refractory if that doesn't tell you enough pull the MSDS
online. A MSDS won't tell you the formula but it will tell you what's in it.

I line my propane forge with Kast-O-Lite 30. It's a high alumina ceramic,
bubble refractory. High alumina doesn't care how much caustics you apply to
it it's commonly used industrially in 3,000f+ ammmonia atmospheres furnaces,
2,600f borax is baby shampoo by comparison. The "bubble" part are little
tiny vacuum filled(?) ceramic spheres seriously decreasing it's thermal
conductivity. It's a MUCH better insulator than your typical castable
refractory say Missou. I have ZERO bad to say about Missou, it's good stuff
I just don't use it, I THINK the Kast-O-Lite is better.

Like most water setting materials the less water you can get away with the
stronger the set product will be. Still, if you're troweling it on it has to
be wet enough to trowel, just do NOT get carried away with the water. Less
is best! Follow the directions as closely as your application allows,
especially setting and curing, it's a water set so it behaves very much like
Portland Cement concrete, it wants to be kept damp or in a humid environment
till it cures. Yeah it wants to be just barely wet enough to work and then
wants to be kept wet while it cures. You guys who work concrete will know
about this kind of stuff.

What you're looking for is about  1/2" flame face, more is okay but much
less and you need a different application technique called a "Kiln Wash." 

When I built my first double lined forge I used Sonotube (concrete form
cylinders) to make the flame face. The outer tube had an inside Dia. the
same as the OD of the hard liner I wanted. The inner tube had an OD the same
Dia. as my target ID for the forge. I hot glued them to a board in position
and rodded a castable refractory into the annulus. Once it was set I pealed
the outer Sonotube and after a couple days dropped a couple lit charcoal
briquettes inside to burn out the inside Sonotube. I let it cure for a long
time with a light bulb in it I was working out of town so it was easy to let
it be.

I had my hard inner liner (flame face) and a pipe shell so I wrapped the
hard liner with Kaowool and taped IIRC 3 layers of news paper around it to
act like a ring compressor and slipped it into the shell. The first firing
took care of the news paper nicely.

Anywho that little digression is just an example of one way to make a double
lined forge.

Okay, lets assume you have your inner liner and backer in the forge shell.
Did I forget the floor? Nope, don't use fire brick, it isn't resistant to
hot fluxes and hard fire brick is a terribly good heat sink with an R value
of less than 1. Soft fire brick is REALLY fragile even if you don't weld on
it.

Instead use high alumina kiln shelf. The stuff is strong, is very resistant
to chemical erosion at temperature, has a low thermal conductivity # and is
about the same heat sink as the hard liner/flame face.

Kiln shelf is my recommendation for a forge floor, cut it with a carbide
(veneer) or composition circle saw blade. Or you can score and snap it like
glass or brick. Or if you're not confident about cutting it yourself have a
ceramic supply store cut it for you.

Okay, the last step, ITC-100 or equivalent, there are other products as
effective and a LOT less expensive. Plistex and Metrikote come to my dented
brain. Mix according to directions, wet the liner and brush or spray the
kiln wash on, coat thoroughly and allow to dry according to the directions.

I've been experimenting with adding Zirconium Silicate flour to castable
refractory when making my flame face (inner liner) and not buying a kiln
wash.

Zirconium Silicate is THE magic ingredient in ITC-100 it's truly amazing
stuff is the next hardest thing to diamond, has a vitrification temp around
4,500c and makes cool things like knives, hammers, etc. 

Anyway, that's the basic of a forge liner with good performance and about as
long lasting as you're going to get without spending stupid HUGE $ on exotic
refractories.

Contact Wayne Coe for small quantities of forge building materials at a
reasonable price. There are also detailed instructions for building forges
and burners available on his site. We just do it a little differently his
methods are sound.

Wayne Coe
www.WayneCoeArtistBlacksmith.com
waynecoe at highland.net

Now about Burners. Some of you are familiar with me and the T burner. It's a
Naturally Aspirated (NA) propane burner that's easy to build and tune. Gun
(blown) burners simply use a blower to supply combustion air rather than
induce it with a propane jet.

There is no real difference in performance between NA and Gun burners, how
hot your forge gets is simply a matter of how much fuel:air in the proper
ratio burns per second in the furnace chamber. PERIOD. Both types have
advantages and disadvantages, just like everything, those arguments degrade
into a blacksmith version of the Ford Chevy argument so use what you like, I
don't have a dog in the fight.

How you aim the flame in your forge does make a difference on a number of
levels and there are a number of factors determining which you want or need.
The important factors are what you're doing and chamber size. A common
alignment is straight down at the center of the forge floor. My shop forge
is aligned this way. It makes a hot spot directly under the burner and the
chamber temp falls off with distance. I use the differential so certain
areas of my work are more malleable. This allows me to work on one part
without having the rest all floppy hot and need tweaking later.

Guys who do a lot of welding, especially welding billets need a more even
chamber temperature. In this case you want either a number of smaller
burners or a burner aligned tangentially to one forge wall to induce a
vortex. The spinning flame will even the chamber temperature significantly
better than any perpendicular alignment.

Cylindrical with a flat floor, or as I've started calling them "Vault"
shaped, chambers are good for supporting a vortex UNLESS you align the
burner to aim straight down at the floor. In that case the flame spreads and
when it impacts the walls it's hitting a pocket and stalls. Sort of. If you
align the burner so the flame impinges the top or one wall at a tangent the
flame has no choice but to form a strong vortex in the chamber. Behind the
burner port will be a low pressure zone so as the flame wraps around the
chamber it impinges the same pocket that traps the perpendicular flame but
now it's a low pressure zone so the flame turns up and goes on around.

I need to find someone who can do cgi animations a picture is all you need
to see all that talk.

Anyway, aligning the burner to direct the temperature where you need it is
important and easy to determine experimentally without building a permanent
forge. Yes boys and girls I'm talking about the ever loving "brick pile"
forge. This is one reason why I have a couple dozen fire bricks, hard and
soft under a table. I can experiment with chamber shapes and burner
placement without casting them in stone. They're not long term so I don't
really care if they're efficient forges I'm just looking at temperature
patterns, cold spots, etc. 

The other real benefit of a brick pile forge is doing demos. Most of the
guys up here burn propane, decent smithing coal is a serious expense if you
can get it at all. Anyway, I like to hope the occasional spectator at a demo
will give it a try. However buying a forge is more expense than most folk
are willing to invest just to try something out and visiting my shop isn't
the same thing. $50 in fire brick, $15 in plumbing parts and another $50-60
for a reg and hoses is within most folks test drive range.

Long and rambly I know, any of you who read me on IFI know that already.
I've been making T burners for some time now and since Mike Porter started
posting on IFI we've been having some pretty in depth discussions of burner
and forge design. Besides I LIKE playing with fire. ;)

Frosty

-----Original Message-----
From: TheForge [mailto:theforge-bounces at mailman.qth.net] On Behalf Of Daniel
Kretchmar
Sent: Wednesday, August 17, 2016 3:59 PM
To: Blacksmithing List Sponsored by ABANA
Subject: [TheForge] Kaowool vs Soft Fire Brick

I have questions for the hivemind:

I wish to build a new forced air forge.  My current plan is (starting from
the outside) 1. A shell from a 40 lb propane tank 2. A layer of fitted soft
brick with high temp mortar or a layer of kaowool 3. satanite 4. Itc-100

Questions: Will softbrick insulate as well as kaowool?  Will soft brick be
more durable when using borax for flux or will it melt just as easily?

Suggestions are appreciated.

Danr
www.,irontreeworks.com
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