Starting build with 30 gallon barrel

First test smelt. Temp probably north of 2500 F. Had to break out the welding glasses!




Took less than 15 mins to get to desired temp. Only needed propane. Only about 3 psi! Kept at temp for 30 mins. I'm pretty certain that I can get to iron smelting temps on propane alone. Still, diesel or kerosene would be cheaper.

 

I made one significant change to furnace that greatly improved its performance. I added a 2 inch high pedestal, 8 inches in diameter, on top of furnace floor. Plinth goes on top of this. This created a 2 inch wide race track around furnace bottom that directs flame. Gave me a PERFECT vortex that covered entire wall from top to bottom evenly. Flame also followed contour of domed lid projecting heat to top of crucible. Crucible heated completely evenly.

Tried to get picture of vortex before furnace became too hot/bright. Pic shows flame hugging furnace wall. You can see floor, part of added pedestal, and top of crucible. Hard to see, but pic also shows part of flame as is circles top of crucible.

Edit: I should add a note here about fuel-air mixture. Smelting typically requires a reducing environment, so best to put max fuel that input air can support WRT combustion. If you run too lean, you run the risk of trying to smelt in an oxidizing environment. This is why pic shows so much flame.

 

I do not know how you are assessing tremperature---perhaps by color alone, maybe an optical pyrometer? But if a person is doing it by eye, one pretty reliable sign for me has been to observe when the pool remains clear of scum if not permanently, at least for "quite a while." Below 2525 or so the pool will scum over rapidly right after or as you are skimming. at 2550 or so the pool will either remain permanently clear and reflective or it will scum only very slowly. As you also point out the metal will be quite bright and nearly white (maybe a hint of yellow) in color at 2525 to 2550. If it is orangish it is below 2475 for sure. Complicating the color estimation are ambient light conditions, obviously. Commercial foundries use immersion pyrometers for a reason even though they pour iron 5 to 7 days a week.

Good job on getting your furnace good and hot. Not bad for your first melt! Had to feel good.

Denis

 

Now that I've got furnace tuned in, I need to find a better way to measure temp at high end of range, between 2500 and 3000 F.

Really appreciate your insights on temps. That helps a lot.

Currently, I estimate temp based on color. Once interior of furnace is white hot and I need to break out the welding glasses to look inside, I'm pretty confident that I've hit at least 2500 F. When smelting iron in crucible and color goes from white hot to brilliant/insane white, I assume that I've crested 2700 F. Typically, I'll start seeing sparks coming from crucible kind of like a sparkler once I'm near iron smelting temps.

 

Would it be possible to do a sketch of your pedestal and plinth assembly with rough dimensions?. I'd be curious about the layout you use and how small the race track is.

 

Platform and plinth are round in reality, more-or-less. Perspective gives the illusion that they are ovoid. Platform is floor from my old furnace.

Excluding domed lid, furnace interior is 12x12 inches. Platform has 8 inch diameter and height of 2 inches. Plinth has 3 3/4 inch diameter and height of 2 1/2 inches. Gap between furnace interior wall and platform is 2 inches. Internal diameter of pipe is 2 inches.

Domed lid is not shown. But it has 14 inch diameter and height of 6 inches.

 

That matches my mental image from your description but I wasn't quite sure: so it seems overall plinth geometry plays a significant part in steering the flames into a swirl/circle.That setup is both opposite and similar to what I'm currently using which is a larger disc sitting on top of a smaller diameter plinth cylinder. I'm planning to ditch the disc and run with the new narrower tuyere (narrow for me) but it looks like I'll need to cast a wider plinth than my 5"/128mm one to be sure to get decent swirl.

 

So, here is what seems to be going on, I believe.

The 2 inch gap between furnace wall and 8 inch diameter platform acts as a guide to help keep flame within a horizontally constrained path for longer than it might otherwise. This constrained path increases flame velocity and gives it at least one full revolution before rising.

Having fuel enter furnace interior at an oblique angle such that it hits interior wall almost immediately after entering chamber is at least as important, perhaps more so. The sooner the flame starts skimming the furnace wall surface during its first rotation the better. Just like skipping a stone on the water ... in a way.

The longer flame skims furnace wall, the more heat it imparts to refractory. Radiant heat from refractory can heat crucible evenly if furnace walls heat up evenly.

Lid geometry induced airflow is another important element in even, efficient heating. In my case, the domed geometry of my lid directs heated air around top of crucible in a tight vortex. Actually, induces a “dust devil” inside crucible. Guessing that this has the effect of allowing heated air to remain in furnace a bit longer before it escapes through vent hole.

 

There is much theorizing and testing when your furnace doesn't work as expected. I agree with having a longer flame path to increase dwell time of the combustion as the 4"/100mm tuyere pipe in my oil fired furnace had such a slow flame rate that the flames would travel a short distance of a third to a half a turn round the chamber before doing a right angle turn and exiting the furnace vertically. The oil didn't have time to combust fully until I stuck the refractory disc on the plinth to constrain the flames into a circular path. It did bring the flames a full lap back to the fuel spray which sped up combustion significantly and also evened out the flame distribution as a bonus which then did a turn or two before exiting. I get the same mini slag tornado on top of the crucible which I'll lower a few inches to increase the gap between the crucible and lid. My ideal furnace would be made of transparent fused quartz so you could study the combustion changes as you work.

 

Hrmmmmm... Interesting. I wonder if a synthetic quartz crystal could be grown in the shape of a furnace? That'd be awesome!

This is a video about creating synthetic quartz crystals

 

Whether you realize it or not, you just discovered the importance of having a properly sized furnace for the crucible.
That is the reason for buying a crucible and building the furnace to match. Two inches of space for oil, one inch for gas.

 

There's various objects made from fused quartz for industry so it might be possible to get hold of a 12" diameter quartz tube to make a furnace chamber from or even some windows into a furnace. It'll have to wait until I win a lottery.

 

Just wanted to share pics of the results from my first test smelt. Since this was a test, used low quality material.

The objective was to see how long flux, collector metal, and sample would take to fully melt and determine amount of fuel required. Collector metal was 1 oz of silver. If everything went as expected, I'd retrieve the collector metal from slag.

Interestingly, I apparently had silver-gold alloy instead of pure silver. This was surprising since I smelted very little ore. I collect the ore for its iron content, not precious metals. Didn't expect more than trace amounts of any precious metals... if any at all.

Don't get me wrong. Not saying that my ore contains an economically significant amount of gold. However, it does apparently contain more than just trace amounts, which is pretty cool.

Check out pics below. They are a progression. First pic is material poured out of crucible into "cone" mold. Next is collected metal still in situ. Then, collected metal after being freed from slag. Finally, close up of metal.

 

looks like a need a bit of a redesign. Temp gradient is a bit crazy. Went to smelt some iron and ended up vitrifying my plinth a bit Its made from mizzou.