Refractory coatings, cast-ables, etc. what would you do and why?

[javand]

Ok guys, I'm in the process of building a forge, and I've got the basic details worked out. Inswool, burners, etc. etc. no problem, but the deeper I dig into the murky waters of refractory "coatings" and "castables", the less certain I become.

I'm one of those people that's not content with "good enough". I'm not a robot perfectionist, but I like making very informed choices, and when it comes to this area, there seem to be ALOT of options, and very little solid information.

I'll be using an inswool lining, that's easy, but after that, what's the creme for durability and efficiency?

Satanite
Mizzou
Bubble Alumina (Ellis variety)
Kast-o-Lite 3000
AP Green #36

Those are just some of the regular options. Then there's more exotic stuff like Kaolite 3300/Plicast LWI 606, to name a couple. Which I suspect are the same or similar to what Darren sells as "Bubble Alumina" which seems to be a generic name for a certain range of extremely high alumina refractories.


Obviously the generic options are to either cast a liner which is very durable and flux resistant, but not very heat efficient, or use a liner of inswool (or equiv), coated with various combinations of the aforementioned products which yield differing levels of efficiency vs durability and flux resistance.

Confusion comes in some forms from the delineation of "castables" and "coatings", of which I think many of the products labeled "coatings" are still castables. Where as things like ITC-100 or Plistix might be more adequately separated as coatings specifically.


Obviously I know I'll be using an Inswool liner, but the "coating" options are where the possibilities start seeming endless, without knowing more of the rules.

I've heard of people soaking a layer of inswool for instance in mizzou. The standard option seems to be using Satanite for the top of the forge and "bubble alumina" for the bottom, and coating the whole thing with an IR coating like ITC-100.

Would it be better to coat the entire thing in Bubble Alumina or an equivalent Ultra-high alumina product like Plicast LWI 606 (~95% al2o3), then coat or not with ITC-100 or Plistix? Should I soaking the top layer (I'll be using two 1" layers) of Inswool in some concoction and then coat over that with a separate product?

Should I use Mizzou on the top and Bubble Alumina on the bottom, etc. etc. etc?



What do you believe the absolute best combination of materials would be to reach peak thermal efficiency coupled with peak flux resistance if building your dream forge and economy wasn't an issue? Why?


I know it's long winded, sorry, but I'd really like some knowledgeable input on this. Please hold the "just use satanite and a firebrick cause it's cheap and works fine for me" comments, I'm already aware of that option, and may end up doing that, but I'd really like to know what the full scope of possibilities are and why one is better than an other..


Thanks!

Javan

 

[NickWheeler]

I can understand your approach, but in a nutshell, you are over-thinking it. Over-engineering if it makes you feel better

Unless you set up several forges in the same shop that have all the various coatings, it's going to be hard to get any kind of definitive answer as to which exact combination is going to yield the absolute best results. Most combinations of materials are going to give good results. Having a good overall forge design with a well made burner will make one of the biggest impacts on your heat and efficiency.

Bubble alumina will not work (IMHO) to coat an entire forge. It looks like packaging styrofoam that's been broken up into all the tiny individual polystyrene beads. When you mix it, it keeps that form, it just sticks together. It does NOT spread well. I have it in the bottom perimeter of my vertical forge----where the CeraChem (2600 degree *improved* kaowool) walls meet the kitty litter floor. It was difficult to spread the bubble alumina even in that bottom corner.

I have two inches of CeraChem on the walls and ceiling of my vertical forge which I misted with a water bottle, applied a coat of Satanite, let it dry out for a bit, applied a second coat and let it dry out, and finally applied a coat of ITC-100. This has been holding up well, and is VERY efficient.

I've had several forges in the last 10 years, and this combination seems to be the best marriage for trying to bring together the most efficient and the most durable lining.

 

[Salem Straub]

I was gonna tell you the same thing, basically. Nick is right on with 2" of blanket, satanite, and a top coat of ITC 100. As far as a dream forge with the best efficiency and flux resistance, my dream forge would have to be two or more forges, used for different purposes. I like a horizontal forge for HT and blade forging, and a vertical forge for forge welding. The vertical I'd build for maximum flux resistance, the floor in these is easier to replace anyway. I think flux will eat almost anything eventually at those temps, so you want to have something replaceable as the floor. Kitty litter, as Nick mentioned, is a common and effective solution over the floor. I use a pad of replaceable bricks that the shell sits on.

As for the HT/forging forge, just don't weld in there and you won't have to worry about it.

 

[javand]

Thanks Nick,

That's the kind of information I'm trying to find, I had no idea the consistency of Bubble Alumina, and couldn't find it elsewhere. What you say regarding that makes sense.

I'm just trying to make an educated decision, in the end my forge will likely not be over engineered, but I like to start with an extensive knowledge-base. Normally I can gather enough information without asking, but this is one of those cases where it's hard to find much real data.

I'm curious though that maybe I was dead wrong about the similarities of the Kaolite 3300/Plicast LWI 606, because the limited information on these that I can find list them as insulating castables.

 

[NickWheeler]

I totally understand Javan. I should have pointed out the reason I know you're getting caught up in small details is because that is what I ALWAYS do. Trying to educate yourself on it should be commended--- I didn't want to be poking at you for trying to research it. Afterall, a small heating unit inside of a garage/shop that can reach temperatures of 2500+ degrees is not something to take lightly.

But a forge is one of those things that if you wait until you have THE PERFECT forge figured out----- You'll never get around to actually having one.

 

[Makermook]

I lined the inside of my 8" diameter by 14" deep forge with 1" of kaowool and coated it with about 3/8" of satanite. I've had it at welding temps for considerable amounts of time with no problems at all. Well, I did put a sacrificial firebrick on the floor of the forge to catch flux and such, but I haven't even had to change that out yet.

I spent a lot of time worrying about which exotic coatings to use and finally decided to go with cheap...so far, it has worked well for me.

 

[JCaswell]

IMO the best forge is all solid refractory (cast and fire brick). No ceramic fiber.


There are two reasons:

1. Ceramic Fiber is dangerous. It's a known "probable human carcinogen". If you use it in your forge you WILL be breathing it. It degrades and the microscopic fibers are blown into the shop as the fire blows on it, not to mention bumps and scrapes of workpieces going in and out. Rigidisers and Coating helps until it's damaged or wears. You WILL be breathing it.

2. CF reflects heat very well, but provides no real thermal mass. If you're just forging for shape or are doing simple PW, it's not a big deal, but it really is a benefit to have a big bank of heat when doing trickier, high-stakes stuff. This is because once your forge is heated, you can back way off the blast of fuel and air beating on/around your joints. This promotes a much better atmosphere for your material and flux.

I run natural gas and reach welding heat with two burners in about 35-40 minutes. It takes a little longer to soak the relevant refractory for best effect, but once there, I can throttle back to nearly off and maintain welding heat for hours. It seems to work like momentum, if that makes sense.
I usually let it blast away full-bore while doing billets with big cross-sections, but when it comes to doing long welds (which will require maybe a dozen consecutive heats) over several surfaces simultaneously in open air, the welding atmosphere made possible by ample thermal mass is an advantage.:thumbup:

... Not to mention what they lose is portability they make up for in durability!

 

[Stacy E. Apelt - Bladesmith]

There are a few engineering rules that can be applied to a forge to make it better. Mr. Caswell and the others hit on some of them.
1) the thermal mass of the liner is what will hold ( and thus radiate to the blade) the heat.
2) The amount and quality of insulation is what will allow the thermal mass to hold its heat.
3) The more robust the forge - the longer it will last and the better it will work.
4) A forge is just a fire proof box until it gets a burner. The burner design is often the most skimped on part of a forge.

The shell of a forge is nothing but a container for the insulation and liner. It should be sturdy and keep the insulation covered and contained.The standard is a piece of pipe or a used metal container .Everything from plain pipe up to a 55 gallon drum has been used.....if it won't burn, it will work.

The liner is normally a built up layer of Satanite between 1/4" and 1/2". A coating of ITC-100 over it is standard ( and will make the lining last longer and work better). The insulation is normally made stiffer by applying Rigidizer and then smoothing on several thin coats of the satanite. After the satanite has been fired and cured, a coating of ITC-100 is applied and fired to finish the liner.
A cast liner from Mizzou or a similar product is normally about 1" thick. This needs to be insulated from the outside to hold the heat in, or it will be radiating both ways. A wrap of 2" Kao-wool is the norm, with the shell around that. Whether the forge has a satainite coating or a cast liner, 2" Hi-Z wool is the way to go for the insulation. A mentioned, the wool needs to be sealed in the forge. All exposed places should be mortared with satanite to keep the fibers from getting into the air. Rigidizer is helpful for this, and makes forge construction easier,BTW. The simple way to do the sealing is to make the forge shell a bit longer than the insulation, and add an end layer of satanite to close the wool up. Metal end plates can then be bolted on the shell.

The floor can be covered in Bubble Alumina or have pieces of hard fire brick mortared in place with satanite. This will resist the flux from welding eating up the liner. It can be replaced as needed.

Lets talk about how a forge works to understand how you should build one:
Your burners need to be able to heat the forge liner and bring it up to heat, then the heat is radiated to the blade. If the burner is only supplying the heat directly to the blade, you might as well be using a rosebud tip on a welding torch, and forget building a forge. Make the burners as good as you can. Blown burners with some system to control the air volume ( VS , 2" gate valve, or a choke) will be the best choice. a mixing chamber in the burner design will greatly increase efficiency. That entails having the pipe size increase by 100% or more just before the pipe necks down to the burner flare/tube. If you are building a blown burner with 1" pipe, have a short section of 2-3" pipe in the middle.....it is that simple.

The liner should be insulated from the outside and radiating heat on the inside. If you insulate the liner with a layer of bubble alumina or other material inside, you are slowing down the process and making the forge less efficient. The floor has to be protected if the forge is used for welding, but other than a layer of ITC-100, nothing should be between the blade and the liner sides and roof.


The forge liner should be allowed time to heat up to the desired temperature from the flame, and then the flame is reduced to a point where it maintains that heat. A forge that heats up to welding temps in five minutes will use a lot of gas to try and hold that temp....if it can. It takes time to transfer all that heat into the thermal mass. A PID controlled forge will do this automatically. For about $100 extra ,you can build the forge with PID control and be able to do HT in it with great accuracy. Two-stage PID control will also adjust the air volume as the PID cycles.

So, as far as the most "bang for the buck" in a forge:
Two stage PID control
1" blown burner(s) with VS air supply ( from a true forge blower)
2" insulation, properly sealed
1" Cast liner with ITC-100 coating
Firebrick floor
Robust shell that can be welded to (legs, burner tubes, end plates, etc.) A section of 12" SCH80 pipe works well for a basic forge.

Final caveat - A forge is just a hot box.....it won't make good knifes by itself. Good knives comes from skill and practice. Good knives have been made in campfire embers, and bad knives have been made in a high tech forge. It may be the best to build your first forge as a simple 8" tube forge with a single 3/4" burner...... and then when you have gained the skills, sink a lot of money and time into an over-engineered, heavy, and non-portable uber-forge.

My latin is rusty, but it should be something like this;
Ambulo ante curso

 

[Karl B. Andersen]

I say just build the forge in the first place to facilitate ease of re-doing the forge every few years.
Here's mine I just re-did after two years of Damascus forging, as well as all the other knives to support a full time knife living.
Top and bottom remove with bolts.
Inside looked like it did the last time I lined it.
This is after coming up to heat for 15 minutes.
Where my palm is was 55 degrees, and on the top was 57 degrees.
One layer of wool over the entire length and two layers up to the bottom of the window.
Two layers in the top.
Coated with about 3/8" Satanite.

 

[JCaswell]

Here an OLD picture of my forge--pretty much the final evolution of a specialty forge I designed for general versatility via easy configuration change. I've been using it for 11 years.

The base is solid cast refractory ... about 4 inches thick.
The top is also solid cast refractory with burners I pulled from my old Mankel (they worked well enough, so why build new ones?)
This VERY HEAVY top rests on fire bricks which can be easily configured to provide specialty shaped/sized forge interiors, plus they're easily replaced when needed. They can be stacked to produce a taller interior too, if needed.
The work-rest bars on both ends telescope out when needed to support long workpieces or short ones with long handles.
The blower manifold is extended to protect the fan from forge heat especially while the forge is configured with openings on the sides.
This runs on natural gas and no matter how much I run it, my Gas bill, which is calculated in 'tiers' never seems to get out of the lowest/base tier, so it basically runs for free.:thumbup:

 

[javand]

Thanks for the replies guys.

FWIW, if it seems like I'm putting the cart before the horse, or whatever cliche applies, this isn't me building my first and only forge. I've got a charcoal forge that'll easily reach welding temps that I"m forging in daily, and I've been regularly participating in Damascus forging with Zoe Crist lately, using his vertical, which is the traditional forced air, satanite coated ceramic fiber variety.

So, no amount of time wasted in research keeping me out of the forge, nor do I think that any level of forge quality is going to enhance my end result. I'm building a gas forge for convenience mostly, and for the ability to use at my studio, where having a coal or charcoal forge just isn't an option.



Stacy raises a good point, one that I had been considered about but never see mentioned in anybody's builds: Casting a full insert, and insulating that with CF. Mostly I see people use one or the other.

It's also good to hear what I knew, but never see mentioned about castables, being that once you bring them up to heat, they can be efficient if used properly.


I'll definitely be using propane, we live in the country, and even though my studio is in town where the gas forge will likely reside, I don't want to be tethered to that infrastructure.



I do have a question about forge design though, is there any draw-back to using a forced air style burner for a horizontally oriented forge? Most of them I see are use venturis, where as most of the verticals are forced.


I'm leaning toward making a horizontal, I've already got a horizontal compressor tank for the body, 2" cerachem, or inswool htz insulation, with a cast mizzou inner. ITC or plistix coating that. Last question, could I coat the bottom half of the mizzou core with bubble alumina or equiv for even better flux resistance?

I understand that I'll need to cast some "legs" onto one side of the mizzou inner to keep it from compressing the ceramic fiber.


Good info guys, thanks!