Forges 101

[Mikey98118]

Yes.

[Mikey98118]

I have noticed a recent uptick in activity on this thread, but fail to see a corresponding increase in questions. Your worries aren't state secrets, or even surprising to us. So you might as well fess up, and get some answers. We all promise to be kind; really we do

[Frosty]

It's funny, as tired as I got of answering the same questions over and over the recent slow spell is kind of sad. Reading the recent posts I see we have a fellow in Ymber who has indeed read past posts and only needs a little clarification. 

Kind of cool eh?

Frosty The Lucky.

[Trevor84]

Oh don't you worry, I promise that I will come running with some confusion soon enough... 

I started school last September, college classes after 20 years out of school. I hadn't even completed grade 10! Through disability I was blessed to have financial support, (even if I did have to debase myself and fight tooth and nail). 

I am taking the human service worker certificate, its a one year course but that's BS! Every instructor says that it should be a two year due to the course requirements. 

 

Blah blah blah! 

 

The first semester was rough but got me two B's and a C. 

 

I just finished two heavy classes (counselling strategies and case managment) A- and a B+! 

 

I struggled, from chronic pain from degeneration and constant muscle spasms and then the massive emotional journeys through my life. LoL, 

 

 

Yuup, human service worker, blacksmith, rocket tryentist. 

 

I hope you are doing well aaaaand I am working my way through redesigning my little shop. I am almost finished my 2x72 grinder and then I will be servicing the old ir ovens! 

 

You happened to catch me at the perfect time... I just walked into the shop full of excitement and aspirations for some productivity. 

Now I feel obligated to to hurry up and get caught up now that I blabbed about new tools and shop designs. 

 

Ear plugs are going in, smock is going on and my goals are specific! 

Sincerely, Trevor (Lone Tree Forge) 

 

Ps, if I don't reply for the day it is because I have become hyper focused and on task. 

 

[Mikey98118]

16 minutes ago, Trevor84 said:

rocket tryentist.

Good one, Trevor; I like that

46 minutes ago, Frosty said:

It's funny, as tired as I got of answering the same questions over and over the recent slow spell is kind of sad.

Yes, we just never seem to be satisfied...I have had some of the same problems. We must do a better job of coaxing?

[Frosty]

I see can folks devoting a life's pursuit of the tryintific method. You are the Father of Tryence Trevor! I'll be honored to be able to say I knew you back when. 

I'm thinking we need to come up with something new that nobody understands so we can explain and describe it to folk. How about a coal gun burner? Guys would need to make crushers to powder it and feed mechanisms before they could start on the burners themselves. Hmmm?

Frosty The Lucky.

[Mikey98118]

 

Whether you build or buy your gas forge, it is likely that you will need to provide your own sealing and heat reflecting coating on its internal surfaces.

Hot-face materials:

Plistix 900 is a fire clay powder consisting of 94% aluminum oxide, and 1.7% silicon oxide, with 1 to 5% aluminum phosphate as a binder; it is in many ways the premier example of a thin hot-face seal coating and is recommended for use on cast refractories and ceramic fiber blanket (but not over ceramic fiber board), it is a general service sealant that forms a protective thermal barrier for ceramic fiber blanket insulation; it air sets to a hard surface.

    Plistix is sold as powder, and is mixed with tap water to a consistency of sour cream; then dabbed on from a disposable paint brush in 1/16” thick layers (each layer will coat 1.2 square feet per 1lb. of powder).

    After application, seal equipment interior with plastic, and allow to dry slowly at temperatures above 60 degrees to a hard set. Post drying, fire the first layer and all subsequent layers can be applied with brush strokes, prior to drying and firing. Bring the coated equipment up slowly to temperature, to avoid cracking and other damage from thermal shock. Air dry and fire every subsequent layer before adding additional coats.

Kast-O-lite 30 insulating castable refractory can be troweled or cast in a 1/4” to
 1/2” thick flame face layer, giving a large measure of thermal protection, along with mechanical armoring from thicker layers, in case you are moving crucible tongs in and out of your equipment, and for general shop safety; it is use rated to 3000 F, is alumina based for flux tolerance, contains mini silica spheres to provide insulating voids, and is very resistant to thermal shock; it weighs 90 lbs. per cubic foot (compared to 146 for standard alumina refractory). Kast-O-lite 30 has been the favorite refractory for construction of home-built forges and casting furnaces for more than twenty years.

    This refractory hardens gradually enough that the edges of equipment surfaces can be scraped with a straight edge, so that those surfaces meet perfectly, allowing two part forges and furnaces to run with little flame leakage.

    Kast-O-lite 30 has a moderate insulating value of great importance for protecting insulation from heat damage; when coupled with a re-emissive (heat reflective) finish layer, it will greatly lengthen the working life of secondary ceramic fiber insulation. 

    Kast-O-lite 30 will stick to most materials. Use cooking spray, Crisco, or car wax on plastic or wood molds as a release agent. I have also used glass jars as forms, and then shattered them by heating to red incandescence, followed by a water quench, after the refractory cured. Cardboard and wax candle molds both burn away conveniently.

Heat reflective coatings: There are inconsistencies found in advertisements for "heat reflective" products; this is a legitimate label, if inexact. when, advertisements go further, and label various products as IR reflecting, they depart from reality. Yes, there actually are substances that reflect infrared energy; the most notable being gold, followed by silver and aluminum. But the difference between cause and effect is important. Actual IR reflectors are only useful as ultra-thin coatings on optical devices, such as light filters in welding helmets, or camera lenses.

    High-emissivity coatings can be used to more effectively transfer heat through a crucible wall (as a thin coating), or to redirect energy, forming a heat barrier in thicker coatings; to illustrate the importance of the point, we will define a typical thin zirconia coating as one millimeter or less (.039") and thick coatings as three to five millimeters and up. The critical difference between a heat barrier and standard insulation is that the higher the heat level the more effective high-emissive coatings become, while insulation typically lose efficiency as heat levels rise. Also, the thicker the coating the more effective a heat barrier becomes. Induction "furnaces" for instance, use crucibles made of nearly pure zirconia refractory, which is transparent to high frequency waves, but is so efficient as a heat barrier that with secondary insulating refractory between the electric coils and an inch or less of it outside the coils, a crucible becomes the whole furnace.

    The way a re-emissive coating works, is that it absorbs heat so redily that it quickly becomes incandescent. Think of a thin layer of tiny zirconium oxide particles exposed to a high heat source, and radiating that energy in all directions; now picture another layer of particles next to the first, with still other layers behind them. Each layer radiates heat in all directions, but the heat source only comes from one direction, so at every additional layer some heat gets subtracted as it is radiated back toward the heat source.

    So, a thin re-emissive coat will transfer lots of energy through a crucible wall, while the portion of heat it radiates back into the equipment is then re-radiated back at it, while the thicker coating on equipment surfaces reduces heat transfer that would otherwise happen through conduction. Re-emissive coatings are a simple but elegant form of recuperative energy generation. By converting combustion heat to radiant energy emission, heat gain from combustion is mostly retained on interior surfaces before the heated gas is lost out of exhaust vents, while heat loss through conduction is greatly reduced, with the added benefit of reducing heat stress on ceramic fiber insulation.

    Efficient heating equipment is designed for radiant energy to do most of the heating work, with part of the combustion heat saved up on the radiant surfaces, so that direct heating from a flame becomes  a secondary heat source on the work. By the time your equipment interior reaches white heat, less than a third of combustion energy is directly heating metal parts or a crucible, while radiant heat is doing most of the work. Once you understand these principles, why movable exterior baffles coated with a high-emissive layer trumps looking for the best exhaust vent size should become obvious.

    Zirconium silicate (zircon) is about one-third silica, so it takes a thicker layer than zirconium oxide (zirconia) to do the same re-emission job, but then it is also far less expensive and much easier to employ. It is a fact that the smaller the particles of zirconia the greater the percentage of heat re-emission they create (as low as 68% to as much as 95%). The zirconia particles trapped in the silicon matrix of commercial zirconium silicate are minuscule.

    Zirconia crucibles employ very crude particulates, and yet they are so effective as insulation that they become the entire furnace, when wrapped in a high frequency coil, and insulated by a further layer of loose zirconium oxide. So, the thicker the re-emissive layer on equipment interiors the better—always providing you use it in a manner that will stay attached.

    Hot-face heat reflectors can be as minimal as re-emissive coatings over ceramic board and rigidized blanket, or painted on a 1/2” thick layer of Kast-O-lite 30 cast refractory. But an armored tile of 5/32” to 3/16” thick made of homemade zirconium silicate “clay” has become a superior option, thanks to Tony Hansen’s famous Zircopax formulas on digitalfire.com: https://digitalfire.com/4sight/material/zircopax_1724.html

    If you are willing to take responsibility for understanding and using raw materials, there are a number of alternative choices, which beat the heck out of commercial heat barriers; not only costing far less money, but sometimes giving better performance at the same time.  So called IR reflectors (actually high-emissivity coatings) will be of especial help in raising efficiency while protecting interiors of heating equipment; let's lay them out.

    The most effective commercial heat reflection coating claims "up to" 90% IR “reflection.” But, "up to" is actually a cover for the nasty truth that their formula can also mean as low as 68% heat reflection; it’s all a matter of zirconium oxide particle size.

    Being a naturally suspicious type, I tried separating the colloidal content from cruder particulates in the top commercial product by spooning some of their thick mud into a water glass, and presto; the crude stuff fell out of suspension in the mixture, and immediately sank to the bottom of the glass. So, I mixed in as much more mud as would separate, and painted the thinned-out coating over a previously coated, and heat cured surface. My forge went from bright orange to lemon yellow incandescence with the same burner and regulator setting.

    When it first came on the market, stabilized zirconia flour cost twice as much as the regular kind. Today, there are three different ways to stabilize zirconia, and the price has fallen to about one-third more than the regular stuff; this is an important factor to keep in mind. So, if the colloidal particulates are so much more effective why have crude particulates in the content? MONEY; what is commonly called zirconia "flour" is nearly 100% colloidal, and will give you the full emissivity benefit; but it's not cheap.

    Zirconium oxide flour is the most effective heat reflector available, but it changes its crystalline structure at yellow heat, from cube to hexagon and then back again during cooling, so twice a heating cycle, it also changes particulate size, which is very hard indeed on every other ingredient in a hard cast refractory; not so slowly turning them to dust. And so manufacturers of high heat crucibles (and others whose products justify the added expense) employ stabilized zirconia, mixed with a binder to make tough refractories and coatings.

Zirconia re-emissive coating: Published government sponsored experiments with zirconia coatings back in the nineteen-sixties tried a number of binders; the most successful was orthophosphoric acid (commonly called phosphoric acid); a readily available and inexpensive product that stays suspended in water; it has some interesting physical attributes. When painted unto a surface it is adhesive, and will hold zirconia particles suspended on walls and ceilings; when heated, it polymerizes, as the acid forms esters. Thereafter it remains on the surface in a vitreous (glasslike) form at room temperatures, and becomes soft and very adhesive above 365 °F (185 °C) from then on. Mixed with zirconia flour this is a highly effective heat shield, but isn’t physically tough. On the other hand, it is simple to repair.

Zirconium silicate (powdered zircon crystal), is a substance that came into popular use, while manufacturers waited decades for reasonable stabilized zirconia prices.

Zirconium silicate, consist of silicate and zirconium molecules mixed in a stable tetragonal crystalline structure; it makes an end run around the size-change problem. Both zirconium and silicate are very resistant to flame erosion; they combine to form a tough hot-face coating. Zirconium silicate starts melting and separating out into its two constituents at 4650 °F (2550 °C); finally, it is only about 75% heat reflective as thin coats (.040”). Zirconium silicate is reasonably priced; if mixed with a binder, you can build up thicker walls of it.

Zirconium silicate re-emissive coating: Zircopax 95% by weight to Veegum or bentonite 5%.

Hot-face formula: This recipe came from a potter’s supply store. It has ingredients that physically toughen, resist strong alkalis, and reflect heat. The (ingredients (by volume) are: one-part alumina hydrate; one-part kyanite (35mesh); one-part Zircopax; half part Veegum T or bentonite clay.

    With such a low heat reflection percentage, zircon doesn’t appear to be the best choice for a re-emissive coating, but its 75% reflection increases with every additional layer painted on. If you want maximum protection for a hot-face layer, or the best high-emissive coating for a crucible, stabilized zirconia flour mixed with a good refractory binding agent (ex. calcium aluminate) makes the optimal choice; it is usually purchased from a supplier like Reade Materials.

There are three kinds of stabilized zirconia. Rather than its previous price of three times the price of plsin zirconia, they are about one-third more these days:

Calcium stabilized zirconia (melting point 4892 °F (2700 °C)

Hafnia stabilized zirconia (melting point 4892 °F (2700 °C):

Yttria stabilized zirconia (melting point 4892 °F (2700 °C):

 

These are my favorite choices for finish coatings, but are far from a complete list of what is available. Anyone who use something else should feel free to list their own favorites.