You wrote "Please check your email..."
A bunch of emails that didn't appear on my computer yesterday are suddenly there today; go figure.
The first idea your forge welding comments brought to mind was the difference between theory and practice, and how much practice changes over time due to altering needs and greater resources.
You wrote "...but the general consensus about gas tubes for knife forging seems so far to be a five to six inch diameter tube about twelve inches long." I'm assuming those figures refer to the internal forge dimensions; that's about what you'd end up with from a thoroughly insulated gas forge in a two-gallon sized disposable Freon or helium (for filling balloons) cylinder.
You wrote "I can see the benefits of having a specialized forge for each specialized application..." One of the great benefits of compact equipment is that it doesn't take up much shop space, therefore permitting such variety.
You wrote "It is difficult for me to visualize the use of ceramic saggers, especially when the goal would be a round 6-inch tube, 12-inches long." Due to the growing popularity of the warm glass movement, ceramic saggars, which are now used extensively by warm glass artists, has a greatly increased choice of available shapes and sizes through stores like Seattle Pottery Supply. Now, the thing about saggars is that, even more than regular kiln furniture (ex. disks and slabs), saggars are built to resist thermal shock. Direct flame impingement is one of the reasons castable refractory and ceramic blanket are so popular with forge and furnace builders. To use saggars as a hot face, two of them would be trapped with their openings together, and their ends would be opened to a desired size and shape with friction cutoff wheels in a hand held rotary tool. Afterward, gluing, insulating and finish coatings would be applied in the standard manner.
You wrote. "on one hand the options described by various folks tends to be daunting, but on the other hand there seems to be a lot of shared features and overlap." You can find people on these groups to clear up any misgivings about the endless sea of advice floating around on the Net.
You wrote "I attended a presentation last winter where the construction of an adjustable size forge and burners were demonstrated. He used rigidizer, a layer of castable and ITC-100." I assume he also employed insulating firebricks; these structures work well enough, but can never equal dedicated tooling in performance; I say this despite my own strong preference for tooling that tends to the universal, rather than specialized.
You wrote " I lean towards 3" of wool for such a forge in the hopes of reducing heat loss." Efficiency is generally desirable, but this should be balanced against practical limits. The major source of heat loss is through exiting exhaust gases. Super insulating is most valuable in large equipment. The the law of diminishing returns "grows teeth" as equ9ipment size begins to be described with words like compact and miniature.
You asked "If I cast a tube that is 6 inches ID, with two-inch walls in the casting, would surrounding that casting with insulation reduce heat loss?" Short answer is heck yeh man! But, only if you reduce the tube's wall thickness to 1/2" ; otherwise you are heating a very large thermal mass within the forge.
But the thoughtful reply would be to suggest you use another building scheme altogether. A lot of guys build their casting furnaces by investing in a bag of their favorite castable refractory (may I suggest Kastolite 3000, or if you're planning to use a really good burner, even the new hotter rated products from its manufacturer HINT, HINT). Build a mold from two different concrete pour tubes (available at large hardware stores), or one pour tube and a large candle. Trap the parts on a board, and make your refractory hot-face. Place whatever structure you desire for an end wall in the bottom of your forge shell, center the finished refractory tube, and pour in a mixture of the same refractory mixed down 50/50 with Perlite as your insulating layer; this is an abbreviated list of instructions. You know, for instance, that provision must be made for the flame passage from burner port through various refractory layers, for instance. And, you may also want to build a flat side into the tube form before pouring, so as to have a built in bottom shelf.
Michael A. Porter
5101 S. Mead Street, Seattle, WA 98118
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