Thursday, August 29, 2019

Adaptable Furnace Base

This piece of equipment a long time coming!

The concept started with my experience at the Scottish Sculpture Workshop in 2017. Unlike previous work there (on the Turf to Tools project) the 2017 visit was intended purely as a personal artist's retreat. A good chunk of the one week was spent with Beth Bidwell in the ceramics studio. I returned to a lost love (wheel thrown pottery) and took part in several kiln firings. This included raku, using a propane fired furnace.

This was a fixed propane fired furnace base, with a moveable top section.
SSW uses a number of high BTU output propane burners, which they can switch out to provide heat for a number of different furnace / heating applications. (Wax burn out, bronze and iron casting, ceramics, special projects.)
Here you see the furnaace base, which is made up from a cut truck wheel rim, lined with castable refractory material. A simple hole in the side directs the blast from the propane burner. This runs on a ventri effect (no electric blower required). Although this base is quite heavy, it is mounted to a wheeled cart to allow it to be shifted around the concrete paved SSW central courtyard.
SSW alumni George Beasley had created the upper kiln segment seen here.
A frame and counterweighted pulley system is attached to a section cut from a metal 45 gallon drum. This containment was lined with k-wool insulation, and had a couple of viewing ports and top exhaust vents cut into it.  The counterweight system allowed the upper to be shifted upwards and easily blocked into place as this was done.
In use, the upper area was set with kiln shelves and supports, containing the previously bisque fired ceramic pieces to be placed. A bit of care needs to be taken at the initial ignition of the propane burner - too much heat, too quickly, will simply shatter the pottery. Over time, the propane feed is increased, eventually bringing the entire upper stack to correct high temperature. The required point is judged by using 'cones' - the heat effect on these observed through a viewing port. *

I was impressed on how basically simple this whole design was.

For a good long while now, I had wanted to set up a larger equipment for bronze casting at the Wareham Forge. I have done a fair amount of small scale metal casting, with lost wax investment (silver and gold), using tin alloy (pewter) in stone, and using the 'green sand' method for bronze. I have been limited in the size of bronze objects by the heat source - with a oxy propane torch. This allows me to effectively head only a small crucible, roughly the volume of a walnut.

Original lead artifact (c 800?) / interpretation in pewter / duplicate in bronze
My intent was to build a furnace that would be able to heat about a litre volume of bronze. At least to start I would continue with the green sand mould making, but this would allow me to cast small sculptures, sword or knife hilts, things like door hardware or boat fittings.

I had originally figured just to build the burner assemblies from pipe fittings, much as I had constructed a number of propane gas forges in use at the workshop here.

Over the winter, Princess Auto had these simple propane burners on sale. (For half price @ $25 each - the included, CSA certified, 10 foot hose normally runs about that much!) I bought three. These burners are rated at 500,000 BTU each.
In January, our 30 year old electric water heater (finally) failed. Although replacing it proved a real pain, it did leave me with the steel tank.
Add some scrounged fire brick I've long had on hand.
For ease of use, apply a heavy utility wagon wheel assembly (also from Princess Auto, purchased a good while back for just this kind of use).

Finished furnace base unit

I had cut (using a zip disk) the bottom roughly 1/3 away from the full tank.
That left me with a short cylinder, having a slightly domed base, all made of 1/8 inch thick steel. The interior is glass coated (think the enamel finish on an older stove), but that will not figure into the construction.
Outside Diameter = 24 inches
Height = 18 inches

Next step was measuring equal thirds around the circle, then cutting three suitable sized holes through the steel for eventual mounting of the three burners. I used the oxy-propane torch for this. ( I freely admit that cutting is something I never have done enough of to really get that good at!). I ended up using a worn down disk on my angle grinder to even out and correctly size each hole.

view down the completed interior
I have a set of half circular pottery kiln shelves, each of high temperature refractory plate, about 5/8 inch thick. One of these I cut to fit as fully as possible over the inside base of the steel shell. In the image above, you can see the edge of the part starting part circle, with the two smaller pieces I shaped to fit into the remaining base. This gives me a durable, hard, flat, fire proof floor. To fill the gaps around the slightly domed steel base, I used a refractory type material I mixed up. I had been saving damaged low density fire bricks, which I broke down to about the consistency of course sand. I mixed this 50 / 50 with powdered potter's clay, with water into a paste. (This using 'Hawthorn Fire Clay' - a fairly high melt point type I use here for building clay cobb iron smelting furnaces.)
As it turned out, the spacing proved almost perfect for the medium density fire bricks I had on hand. Each burner would be centred on one brick, with two bricks spacing around between each burner point. Once the fill and floor plate was installed, the remaining wall height came to the 9 inch length of the bricks (Ok - I did measure this out initially and cut to provide for this.) The resulting interior diameter is 12 inches.
I dry fit all the bricks, to transfer the location for the holes from those cut in the shell. As it turned out (luckily) the holes required were exactly the same size as the largest diameter in my hole cutting saw set. Short work with the hole saw mounted in a drill (cut from both sides to get through the thickness). Given the relatively fragile nature of the fire bricks, this proved a much easier task than I had suspected.
Again test for fit. Each of the bricks was mounted, again using the clay/brick dust mix as filler in the triangular gaps. It is expected this mix will contract a bit (10 %?) as it dries, but given the tight fit of the bricks I don't expect this will present any special durability problem in use.

close up of the master valve and fittings
Figuring out just how to link all the burners together proved a bit of a pain. The tubing diameter for all the pieces on the burners as purchased proved to be 1/4 inch. Maybe a 'standard' size - but not a size that any of my local stores had in supply. Once again, McDonald's Home Hardware in Dundalk saved me. They had just enough fittings, once a set of 1/4 to 1/2 inch couplings were found. This allowed me to get just enough of the (slightly more available?) 1/2 inch steel pipe fittings **
With use of a short piece of 1/4 thread fitted hose I had on hand, I managed to link all three burners together, via one central shut off valve. I further protected the short gas hose by covering it with the plastic pipe seen above.
I kept two of the burners with the original knob equipped flow valves. The central burner (seen above) is 'always on' - controlled by the main shut off. In use, you ignite that first burner from the main valve, then can control each of the remaining two burners individually. This combination should give the greatest flexibility and control over the propane flame = temperature.

The ideal would have been to mount the burners at a slight angle, both off centre along the side of the circular containment and also pointed down slightly towards the floor. This combined angle would server to create a circular vortex inside the furnace chamber, actually improving the flow of heat within. As it turned out, the distance from the brick interior through to the outside of of the shell, with burner mounted, can be seen to be almost exactly the distance to the air input slots already designed into the burner. By maintaining this, there should be the ideal amount of input air to feed the burner as it was original designed.

The completed furnace base unit weighs about 60 lbs. The combination of weight and size, plus the need to be able to store it when not in use in a working shop, lead me to use one of the utility wagon bases I had also purchased at Princess Auto some good while back (again on sale). I welded up a support from angle, which bolts on to the wagon base. The large 8 inch diameter wheels, which pivot on the front, make for good handling over the dirt floor of my workshop. As mounted, the completed unit stands 24 inches tall - a good height for manipulating the eventual crucibles. 

The last step is to take another of the kiln shelves and pattern and cut a suitable lid for this configuration of the furnace base. Ideally there should be a central hole cut, to allow burned gasses to escape. This will also allow two other functions when melting bronze :
1) Allow direct observation into the interior of the crucible (judge melting point)
2) Allow addition of more bronze material during a melt.
Will also need to figure some simple handle mounting to allow this lid to be quickly removed to extract heated crucibles.

Right now I have just used one of the supply hose assemblies that came with each burner. This pulls pressure straight out of the propane tank. There are two possible problems with this :
1) I have 40 lb propane tanks here as my normal supply for the gas forges. I have no clear idea how much fuel this x 3 burner system will draw, or how fast. The larger burners at SSW seemed to function a long time, but that was pulling from a 100 lb cylinder - one per burner. Testing required!
2) As designed, the burner is pulling straight off tank pressure. This is sure to drop (significantly) over the course of a longer session of heating.
I may introduce one of the variable 0 - 30 step down regulators into the system. This itself may need some testing to determine how this effects the overall operation. (??)
Right now I want to give the clay mix a couple of days to completely dry before the initial firing. (Potential steam explosion - and to reduce cracking effects).


Steel Shell = recycled
Firebrick = recycled (but about $10 per brick = $90)
clay = 1/2 bag = $12
floor plate = on hand (but about $55 each = $110)
burners  = on sale (1/2 price) at $25 each = $75
fittings = various = total $55
wagon base = on sale (1/2 price at $60)

To start, I will be making casting crucibles of that same clay / brick dust mixture. I have had good results in the past using the same kind of clay / sand / horse manure mix used for the smelting furnaces. I may end up investing in a couple of modern high temperature refractory crucibles as well.

Next overall part of the project will be converting the upper portion of the water tank pieces into a containment to allow this furnace base to be used as a ceramic kiln as well...

* Use of 'cones' is a 'traditional' method of determining pottery kiln interior temperatures, still widely used in the absence of (often expensive) electric / electronic instruments.
A set of small, thin, pyramid shapes are made of clays with differing melting points. These then are set in a line, lowest slumping cone to the view port, increasing in heat effect towards the interior. Watching each cone slump in turn gives the potter a fairly accurate read on the furnace temperature.

** As it turned out I bought ALL the 1/8 and 1/2 fittings Brent had on hand. Well, Tuesday is order day for the normal Thursday delivery of re-stock. Sorry to anyone in Dundalk who got caught short. (But honestly, commonly I'm 'the only one that buys that stuff'...)

1 comment:

David Robertson said...

If possible have all 3 burners with their needle valves. There is pressure drop between burners and much easier to even out heat with separate control valves. 21600 BTUs per lb of propane. Each burner running full out (unlikely) would use 23 lbs per hour. Total 69 lbs per hour. As a contained heat source probably less than a tenth of that.


February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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