Metal Casting Project Begins

The weekend before last, I finally began the home metal casting project that I started almost exactly two years ago.  Actually I had purchased supplies for it and started construction right after I wrote that article in 2007, but it has sort of been on hold for a while.  The pamphlet from www.backyardmetalcasting.com that describes how to build a flower pot casting furnace called for using a popcorn tin.  I did not have one, so I purchased a steel pale, a flower pot that fits inside, and some sheet metal to create a lid.  The lid was my biggest hold up for a long time.  I've never tried to fabricate anything out of sheet metal before and it was a bit of a trial and error process.  The lid basically had to be a cylinder, several inches thick filled with motar to place on top of the furnace.  The plan called for rivetting a strip of sheet metal to the popcorn tin lid and then filling that with mortar.  I started by cutting out a circle of sheet metal the diameter of the lid.  I bought a cheap pair of sheet metal shears from Harbor Freight to do this and it worked very well.  Then my intent was to cut a strip of sheet metal longer than the circumference of the circle, bend it at 90 near the middle and then wrap it around the circle, rivetting it to the circle.  Obviously this was not going to work because you can't bend a strip of sheet metal into a circle if you have a 90 degree fold in it.  To get around this, I cut slits in the horizontal bend of the strip.  Then it worked, but I discovered that I didn't cut the sheet metal strip long enough to have enough overlap on the edges when wrapped around the circle to rivet it together.  That's where I stayed stuck for a long time.

Then while browsing in Fleet Farm one day I discovered that some time of rain gutter thing was essentially a long strip of metal with the bend already in it and it was long enough to wrap around the circle.  All I would have to do would be to cut the slits in the horizontal edge to allow it to wrap around.  This discovery was enough to jump start the project again.  The only downside is that this metal piece turned out to be aluminum, so it will be interesting to know if it holds up to the heat of the furnace.  Mainly it is just a form for the mortar, and the bottom is still sheet metal and that part will be the hottest, so I'm thinking it will be okay.  If not I can always remake the lid.

To construct the main furnace you cut a hole in the side of the bucket at the height of where the clay flower pot would sit if its top was flush with the top of the bucket.  This hole will be the air intake for the furnace.  Then you fill the bucket 3/4 full with mortar and put the flower pot in leveling it with the top.  You add enough mortar so that the mortar is flush with the top of the bucket and the flower pot.  The mortar called for in the plan is made by mixing 1 part portland cement with 2.5 parts masonry sand.  I couldn't find masonry sand at the home stores so the plan said it was okay to use play sand which I used.  There are many subleties to this process and I strongly recommend buying the pamphlet.

Then you fill up the lid with a mixture of the same mortar mixed with some broken pieces of a clay pot.  I imagine that this makes the mortar a bit stronger since it will be quite thick in the lid section.  You need to leave a hole through the whole lid for a chimney of course which can be done by putting a paper towel tube in the middle.

One thing I have learned from the fire pit project is that regular mortar will not hold up to high temperatures.  Portland cement is not made for high temperature use.  Maybe you could use refractory mortar for this, but the author states that you can get many temperature cycles through the furnace before the mortar will crumble at which time you rebuild it again (or buy a nicer furnace to have even more fun).

Before the mortar dries completely, you need to carve out the mortar through the hole in the side of the furnace for the air intake so that you can eventually drill a hole in the flower pot and insert a tube for air intake.  The mortar has cured for about 10 days now so it is probably ready to go.  I need to finish the construction, and at that time I'll post some pictures.