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42877Re: [hobbicast] Re: Refractory blanket how-to for welding forges.

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  • mikey98118
    Sep 11, 2013
      I'd say that you have it pretty good. The only thing further is that we never discussed what size furnace we're talking about. The ridges might be a little large if the crucible is only 2" diameter, and a little small if the crucible is 6" diameter. You will have to "follow your gut" on the fine points; good luck.

      ----- Original Message -----
      From: Jeremy Winder
      To: hobbicast@yahoogroups.com
      Sent: Tue, 10 Sep 2013 19:34:26 -0000 (UTC)
      Subject: Re: [hobbicast] Re: Refractory blanket how-to for welding forges.





      To clarify what I think you are saying:

      The grooves, for the flame path, should be 3/4" wide and 3/4" deep. Start about 2" to 3" up from the bottom so the flame can make a full circle around the bottom of the crucible before being forced upwards at roughly 20 degree angle. The ridges should be no closer than 1/4" to the crucible.

      Thank you in advance,


      On Tue, Sep 10, 2013 at 3:20 PM,  <michael.a.porter@...> wrote:






      I did not make it clear that the trench depth is supposed to be "just larger" than the crucible diameter; not the wax candle's diameter. The candle diameter needs to be at least one-inch larger than the crucible's diameter, for flame clearance.


      ----- Original Message -----
      From: michael a porter
      To: hobbicast@yahoogroups.com

      Sent: Tue, 10 Sep 2013 02:48:18 -0000 (UTC)
      Subject: Re: [hobbicast] Re: Refractory blanket how-to for welding forges.





      Where does this Porter guy get off with his long winded description of forge building; ain't this a casting group!?!

      Well, this same method can be used to create very efficient casting furnaces, by cutting a trench  in a tight spiral pattern around  the right diameter candle (just barely larger than your favorite crucible); this forces the flame path to spiral up around the crucible, with very drawn out hang time. The method is quite successful, and is in fact patented by a Japanese company.


      ----- Original Message -----
      From: michael a porter
      To: hobbicast@yahoogroups.com
      Sent: Mon, 09 Sep 2013 18:42:31 -0000 (UTC)
      Subject: Re: [hobbicast] Re: Refractory blanket how-to for welding forges.





      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.



      Jeremy - Columbus, GA USA

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