So mike now you have a trip to the dreaded Ikea to buy a large candle. Kathy will be happy to go with you I am sure. You might even be able to bribe her withMessage 1 of 40 , Sep 9View SourceSo mike now you have a trip to the dreaded Ikea to buy a large candle. Kathy will be happy to go with you I am sure. You might even be able to bribe her with dinner in the Ikea cafeteria.Dan in auburn
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On Sep 9, 2013, at 11:42 AM, michael.a.porter@... wrote:
Proposed Castable Refractory Knife Maker’s Tube Forge: Preliminary notes.
The driving reason for interest in solid refractory hot-face layers is the desire amongst knife makers to reach “welding heat.” I don’t argue with progress or freight trains, and so have no intention of joining the debate as to just what temperature constitutes welding heat. Nevertheless, while a forge’s burner can always be turned down for a slower work pace or to handle delicate parts, your ability to speed the work up will be limited by equipment design parameters.
Higher forge temperatures (yellow to white versus red to orange heat) places a lot of stress on ceramic fiber blanket; aging it at accelerated rates. Ceramic fiber coating layers can ameliorate the fast damage rate, but not stop it. When building your first tube forge, you need to decide how hot and how long at a time you plan to run it. If you want a very light weight traveling forge for show and tell, ceramic fiber is the way to go; if you plan on yellow-white heat and constant use, a heavier forge is worth the inconvenience.
It should be self-evident that, whether you buy something to employ as a special refractory tube, and drill it for the burner port, or cast one yourself, this construction method is either going to be a lot more expense or a lot more work than trapping a kiln shelf in place with ceramic fiber blanket.
Mark Feldmann first suggested creating a parts shelf area on cast refractory hot-face tubes, by using a large candle with a flat carved into one area as an internal mold. Furthermore, he suggested that recesses be carved into the flat (in order to cast built in ridges on the shelf), so that an external parts support bar could be aided, or even completely replaced by these ridges.
Previously, the expense of employing a large candle as the inner surface of a castable refractory mold simply wasn’t justified, in view of the countless cheap and/or free cylindrical shapes (ex. bottles) that could readily be employed. However, the chance to come up with an inbuilt “shelf” that is part of the refractory hot-face layer completely changes the benefits available from this expense.
I determined that the ridges should run sideways to the furnace bottom support shelf’s length, and could also be aimed at an angle, to help promote a swirling path so as to increase “hang time” between the flame’s introduction through a burner port (tuyer), and exit from its exhaust opening, in a forge interior.
One of the many things I like about this idea is that it tilts the balance of practicality in favor of cast hot-face sections in forges. A castable refractory hot-face happens to be needed for the popular introduction of Vortex gas burners to portable forges. These burners will, in their turn, completely revolutionize gas forges.
Previously, burners were aimed at a top-down angle towards the kiln shelf on the furnace bottom, as direct flame impingement from a modern burner (with complete primary flame combustion) would rapidly destroy the ceramic fiber forge wall.
Such top-down positioning bounces the flame off the kiln shelf, instead of allowing it to impinge directly on a ceramic fiber wall, and the angling of the flame path permits it to swirl somewhat.
Complete primary wave combustion from high speed tube burners helps reduce deleterious effects on heating parts. Still, a downward aimed flame isn’t going to combust all secondary byproducts completely in the short distance available before is contacts a heating part; therefore, parts will be exposed to some superheated contaminants in a small tube forge, no matter how good the burner is.
Aiming a burner upward, away from the parts shelf on a forge bottom, greatly lengthens the path available before superheated gases touch heating parts, drastically reducing exposure to contamination: it also increases swirling action of the flame path, increasing fuel efficiency. Vortex burners put out larger and longer flames than high speed tube burners; they also should not be placed facing downward, even if you make them easily removable, because sooner or later someone will forget to take them out during shutdown, and the “chimney effect” will rapidly destroy their plastic fans and motors. Yet, the vortex burner represents a great leap forward in burner design.
Unfortunately, aiming a burner upward eliminates the possibility of using a kiln shelf as a flame baffle; thus, a castable refractory must be used in at least a large area of the forge, to form that necessary flame baffle.
As previously stated, casting such a ring is going to add considerably to your workload in forge construction. However, casting a hot-face inner shell that includes a floor shelf and burner port in its form can help add considerable value for your increased effort.
Replacing ceramic fiber insulation with a castable refractory mix: An inner layer of castable refractory is going to need better mechanical support than fiber blanket insulation is likely to provide.
Then too, even with a 3/4” thick castable refractory hot-face as an inner shell, the outer surface (face) of a forge maintaining 2300 F internal temperature on its hot-face (inside surface) will soon reach 2000 degrees on the refractory’s outer (cold-face) side, accelerating fiber breakdown. So, using an insulating castable refractory, instead of ceramic fiber products, becomes worthwhile in the construction of portable (i.e. small) gas forges.
An insulating castable refractory will provide much better support for the molded hot-face layer than fiber products, and if it is made from a mixture of refractory and perlite, you can use one bag of refractory instead of buying two different kinds. Perlite can be inexpensively purchased from the garden section in any large hardware store.
Kast-O-Lite 30 LI (previously called Kast-O-Lite 3000) is rated to 3000 F; it is a tough easily worked refractory, which has more than a little insulating value itself. Kast-O-Lite 32 LI is rated to 3200 F, and is likely to stand up better to direct flame impingement, although the 30 LI should be good enough unless you burn propylene, instead of propane.
I recommend between two and three parts perlite for each single volume of refractory. The typical ratio of furnace cement to perlite is four parts perlite to one part cement, but this is because furnace cement, unlike castable refractory, has no grog content in it.
Recently someone on one of the casting groups commented that there was no such thing as a cement that can have all of the water baked out of it; I believe thisMessage 40 of 40 , Sep 15View Source
Recently someone on one of the casting groups commented that there was no such thing as a cement that can have all of the water baked out of it; I believe this was done in defense of the use of Portland cement in a homemade castable refractory. The official answer would be that the chemically locked portion of water in lime based cement cannot be baked out, but the chemically locked portion of water in refractory cements can be. I have always accepted this official version as the only reality...in the past. The limiting factors on "baking out all the water" are that the refractory must be taken to yellow heat in the first place, and that over time, water content can recollect in refractory if you're not careful to seal the refractory surfaces against water vapor in ambient air.
However, truth of any kind is seldom found effortlessly; including technical "truths". I suspect that a healthy debate, with both sides airing their views, might adjust what the majority of us accept as practical reality--to our mutual profit; this could be important for people wanting to make insulating refractories as secondary layers. Homemade refractory as an insulating secondary layer might be quite forgiving of official standards; standards useful for hot-face layers may constitute a waste of money in secondary refractory layers.
While casually dismissing the Portland cement idea, I have noted both resentment, and a strong hint of "I'll match your official facts with personal experience" in passing (heated) comments from the other side of this issue. Isn't it time they had a FAIR hearing; something open minded, maybe?