Archive for the ‘Furnace Body’ Category

Firing up the Furnace

Posted: October 3, 2010 in Burner, Furnace Body

Well, it’s now time to finally put the burner in the furnace and bring this forge up to temp to do a nice burn-out prior to coating with the ITC-100 Infrared Reflector.  As a result, it’s not going to get as hot and be as efficient as it WILL be later, but I need to get all of the burnout done before sealing it.   This gives me a chance to see what it can do.

This is the furnace after a few minutes to get up to heat.  The object in the center is a steel can that I was melting aluminum in… a nice hot orange.  The rod to the left of that was a graphite rod that I snapped and it fell ito the furnace.  Hot hot hot.


Wrapping up the Furnace

Posted: September 13, 2010 in Furnace Body


As mentioned in Casting the Base, I added in the Inswool blanket around the refractory shell.  At the very bottom, there’s not enough room for the full 1″ wrap, just .5″.  Then 1″, then 2″ at the top.  My last-minute decision to widen the crucible chamber for big pours has reduced total insulation value.  Still, there’s a lot of insulating refractory even with NO extra inswool in the bottom couple of inches (about 1.5″ of highly insulated refractory + 1″ of inswool for the outer liner), so I’m ok, I’m sure.


Casting the Base

Posted: September 8, 2010 in Furnace Body

Before doing the base I had to wrap up the lid.  My initial plan was to weld an iron pipe cap to a piece of electrical conduit that would be an arm to hold the cap (with a hole) in place.   I was having one heck of a time trying to get the cap welded to the conduit.  Maybe it was just a thermal mass issue and if I’d held the heat longer I’d eventually have gotten the cap to sufficient heat… I eventually gave up  as the arm kept getting dislodged, bad semi-welds happening, etc.

Instead, I mounted the lid and created a “basket” from welding rod.  It’s not pretty, but should help keep the arm in place.  Whether my whole assembly is strong enough to support the cap, or whether I’ll need some additional shoring up I’m not sure at this point.   Back to this issue later…

So, I then removed the cap and baked it to sturdy up the paint and drive off water and start burning out some volatiles in the refractory.  I did a nice slow ramp (at least it seemed slow) from 250 to 550 F.  I held at 250 for about an hour then went up about 50 F every 20 minutes in the oven.  The “real” burnout will happen with the burner up to full temp, but at least there won’t be any significant water left to crack it and the paint is now toughened.

So, onto the base…


Casting the Lid

Posted: August 31, 2010 in Furnace Body

Before doing the casting, there was a little more work to do to prepare the cap.  I welded an electrical conduit pivot  on the handle and built up the forge shelf rest to improve strength and improve visual appeal.  The pivot will fit into a sleeve attached to the bottom to allow the cap to slide up and down, and swing out-of-the-way.

To keep the refractory from sliding out of the cap, sheet metal screws go through the metal ring and into the interior.  Additional support screws are located at various points along the surface.  The refractory will surround the screws and the weight will be distributed to multiple points to stay in place and keep from pulling out.

2000 degree F  paint was added to the area around the ports.


Lid Mechanics

Posted: August 30, 2010 in Furnace Body

Before wrapping up the lid, I figured it’d be a good idea to get the mechanics of the lid wrapped up.  Since this is a Foundry Furnace, a crucible will be inserted into the body filled with metal to be melted.  To trap heat inside, you need an insulated cap to the furnace.   So, it becomes obvious that you need a means to move the cap out-of-the-way when you need to insert or remove the crucible.

The simplest method of doing this is to install a hinge on one side and lift it.     This has its down sides though.   If the lid will tilt up, then it precludes us from using the top for other purposes without moving things.   Ideally, I want to be able to use the top as a “burn-out oven” to vaporize wax in investment flasks or to pre-heat molds so that I am less likely to have an incomplete casting.   Keeping the lid horizontal is preferable if at all possible.


Shell Welding

Posted: August 10, 2010 in Furnace Body


Not too much to say here except that I’m a total welding novice, and I finally got some welding done on the shell:

  1. Tacking the steel ring to the lid (in addition to the sheet metal screws)
  2. Tacking the two bottom pieces together
  3. Attempting to weld a burner clamp to a steel bracket that I made

The lid wasn’t too bad.  I made sure to mostly heat the ring and get as little on the tub as I could.

The two bottom pieces could have been worse, but ran into the issue that while the tub SEEMS like a thick piece of metal, it really isn’t… I was having to be very careful not to melt it away as I was doing my welding.  I left a couple of small holes, but by and large it looks ok.   Both of these were simply done in spots, not all of the way around.  The latter would have been nice, but I was running out of patience and time.  🙂

The third one was a colossal problem.  I don’t know why I hadn’t realized it before, but the clamp I was using was NOT steel… I think aluminum.   So, as I was heating the steel the clamp would just melt away.  Bah!  Fine… steel pipe it is.

In any event, here’s the rough-form furnace shell.  Not too horrible for someone that hasn’t welded in 20 years, but not as awesome as I was hoping.  Practice makes perfect.

Over-Insulating the Castable

Posted: August 6, 2010 in Furnace Body

When creating a homemade refractory mix, the common suggestion is to add sawdust or foam beads to the clay and other materials.  The result is a lighter, less dense refractory.  As the refractory is heated up, the organic material burns away, leaving voids in the heat-resistant material.  These voids transfer heat much less efficiently, increasing the insulative value of the refractory.  The net result is that more of the heat stays in the crucible chamber and less leaks to the outside.  For a foundry or forge, that’s a wonderful thing… we’re not buying propane to cause global warming!  

The more heat that stays in the chamber, the faster the piece gets up to melting and forging temps which results in much more efficient operation.  Theoretically, with perfect insulation a candle could melt iron (eventually), but there is no perfect insulator.  As we add heat from the burner, heat is being lost through the shell as well as escaping out the exhaust, which is a shame, but a necessary evil unfortunately.  Still, we want an insulator as good as we can get so that we can add heat significantly faster than it leaves, so that the temperature can keep increasing.  As the temperature increases, so does the rate of heat loss, so ultimately good insulation is the key to high temperatures. 

To this end, most hobbyists skip the refractory entirely and go strictly with a ceramic wool  material that has VERY high insulation value and brings things up to heat quickly.  The downside of this is mainly durability. Physically they can’t take much abuse, and welding fluxes and glass will destroy it quickly.  That doesn’t make them bad, it just means that you need to be careful.  You’d be well-advised to coat the material with Satanite and ITC-100 to give it some substance and  resistance to flux.  Putting a dense board on the bottom isn’t a bad idea either. 



The Furnace Shell, part 1

Posted: August 1, 2010 in Furnace Body

The furnace and shell are two parts of the same whole.  A feature in the furnace makes its way into the shell.  However, since I’m using castable insulating refractory and will need to make molds, etc, I’m just going to break this up into individual elements.

I got my initial inspiration from Lionel Oliver’s Backyard Metalcasting site, specifically his “2 bucks” furnace.  However, I was having a heck of a time actually finding 5 gallon metal buckets!  What used to be prevelant, is now rather hard to come by.  I didn’t really want to buy a ton of paint (and what do I do with it?) just to get buckets, and while ordering buckets online is cheap, shipping them is not.

I contemplated buying some cheap stock pots from walmart, but “cheap” is a relative term, and $30 each isn’t especially cheap though it would have been stainless steel and rather cool.  It may have been a little on the small side too, since I wanted to get an inswool coating in there as well as the castable.  And then, I stumbled on a pair of ice buckets from target that seemed to fit the bill… not huge… a good size and relatively inexpensive.