Hello Everyone, I went to pour some knick-knacks from various bronze plates, bars, screws mostly reclaimed from boats (my other hobby) and had 5 molds freeze up in the runner before they began to fill the parts. Metal seemed 'chunky' on top but nice in the middle at 2100 F/ 1148 C. We were at this temp for a while and decided to pour after removing the chunks. I assumed this crunchy top was dross but am no longer sure. Many screws were discovered in the crucible after the pour, we checked and they were also bronze. Leafing through the Amman brass book, it would seem we were a couple hundred degrees F below melting all bronzes (phosphor bronze is listed at 1300C /2375 F and quick to freeze). Does this seem plausible as a root cause with these symptoms? If we could tune tune the furnace higher (it seemed to be holding at 2100 F), would it make sense to put all the metal back in or just the stuff that hit the molds? I would like to get to the point of reliable bronze, and if it could be with reclaimed metals, all the better. If using new alloy is best for beginners, I can accept that advice as well. I was being supported by the Tamarack crew while this happened so I wasn't allowed to quit or cry or throw tools. On the brighter side we had a phone call with the K-Bond folks at Kent State and improved the sand in the bench considerably and made some successful aluminum pours. I also got to run a vintage vertical mill, eat really good food, help with a community service project, hear many(!) stories, and make a little more K-Bond for at home. Picture of half-filled gating, patterns, crucible, and dross bucket w/ miscellaneous top and bottom chunks.
Looks to be too cold, any pale blue-green flames coming off the molten metal surface in the crucible?. What's the furnace run off, gas or oil?, is it naturally aspirated?. You may be able to get it hotter with a smaller crucible to increase the combustion volume. The flames are from a hardwood block burning on the bronze surface but you can see the crucible is bright yellow, it may be too hot for those cheap Chinese clay crucibles.
Thanks Mark! some more details below, they seem to line up with low temp and low combustion volume. -no ongoing pale blue/greens flames, crucible was more orange than yellow. I got green flames poking it once. -Crucible is one that came with a pouring stand so guessing inexpensive. -furnace is propane gas with an electric blower and igniter system. 100# (45kg) gas bottle was nearing the end but still has 60psi/4 bar (30x outlet PSI) -Crucible was a tight fit, maybe 2-3cm (+/- 1 inch) around top and sides and it was not that full of metal so smaller would have worked. -Plinth was short, also 2-3cm (+/-1 inch) otherwise we'd loose all exhaust space at the top. Another question: is there a better metal for tools to keep the bronze from balling up on them? We were using mild steel. Or is this also just another too low of temp indicator?
Propane burns fast enough to melt bronze in tight furnace confines with an inch or so crucible clearance (see photo above). You may need a higher flow regulator to get the burn rate, like those pottery kiln regulators. Some regulators can be opened and the pressure increased. If the skimmer is glowing red from preheating in the exhaust and the bronze is almost too bright to look at, then skimming should not be a problem. Some perlite can be used as a coagulant to aid skimming the melt.
Thanks Mark. We did not preheat before skimming. Regulator is a cutting torch type (two gauges and large metal valve). Gas output in the burner tube is a straight pipe. Q: Should the bronze dross be thinner/darker/more like aluminum dross once the temps are corrected?
I don't know how the flow rate would compare, we are using a "Red Devil" regulator with a maximum flow of 6Kg per hour or 13lbs, but use nowhere near the maximum flow. We can use a 45Kg cylinder but have a 210Kg that is the main source. The furnace nozzle is a plain 1/2" copper tube entering the tuyere at an elbow so the tube is in the center of the tuyere. https://pinnacleweld.co.za/product/pinnacle-red-devil-lpg-regulator-high-pressure/ Bronze dross is a pile of oxides held together with some metal, so it's not like the bright shiny dross from aluminium.
Ok so the data sheet for that regulator says a maximum flow of 800 SCFH. So if I have 6 Kgs of propane per hour from the red devil regulator, that is equal to 12 litres of liquid propane which expands 270 times to gas which gives 12 x 270= 3240 litres of propane gas which converts to 6875 SCFH. I'd estimate you'd need maybe double the gas flow rate of what you currently have, plus blower air to match. We currently use a 1 Hp centrifugal blower that is choked down a fair bit. The furnace noise is quite loud and can be heard blocks away due to the low frequency rumble similar to a jet engine.
It's not my furnace but the one my foundry mentor uses (as well as myself on occasion) it dates from 1958 and the cross sectional drawing is here: https://web.archive.org/web/20190309210619/http://www.turbofast.com.au/astrotel/furnace1.html It is low efficiency with the outer body reaching 360 deg C or 600 deg F. It's fast to melt due to the high burn rate. We used to use A25 furnaces for bronze but nowadays it's an A16 and the odd A30 for aluminium so wall clearance is of the order of 1 inch per side. I made the mistake originally of copying the dimensions for oil use and could barely melt an A20 half full of brass whereas it works perfectly fine with propane gas. It will run off a 45Kg cylinder for a melt or two but for all day use a 210Kg cylinder is used.
My project's vent is 4"(102mm) burner tube is 2+1/4" ID ( 58mm). We were not using that/my furnace, I was working with Daryl @Uglydog and the gang in Tamarack. I want to say that for the bronze melt in question, the crucible filled the inner cavity with only 1"around the top, bottom, and each side. I also want to say that the burner is a bit smaller than mine and vent ID close to mine but Daryl would have to take measurements with that same crucible to confirm. Here are my dimension with blank lines to use as a worksheet. Mine was designed to be 'roomy' for use with liquid fuels.
So the dimensions are more or less comparable, even the lid opening on my example is similar to your furnace. So if you can get enough gas in it, it should work fine for bronze.
I am glad there is still hope for mine! Daryl has melted brass in that furnace/crucible combination so I am hoping we can turn it up to "11" and get through bronze.
Weird, when bronze is heated to 1200 C - 1300 C (easily achieved in a propane furnace), it does not freeze prematurely, unless you spend too much time for skimming the crucible outside the furnace. When I melt silicon bronze in my electric furnace (which is slightly cooler than propane, max 1200 C), I have no issues when pouring after briefly skimming as it fills the mold normally.
We were only at 2100F / 1148 C according to the pyrometer. We played with the dross/unmelted top layer in the furnace under power but our tools were not preheated so that was also a mess.
Coat your skimming tools with a thick layer of soot from a smudge pot or from a torch running pure acetylene. Your tools will stay clean. Richard
