Using my normal foundry powered by waste oil or gas is fine when the weather is on my side, but the idea of an indoor electric foundry was for a long time very appealing. (See the accompanying video here). There are benefits to an electric foundry:
However, it needs to be understood, they're not fast. If you want fast, stick with gas, oil or solid fuel. But if you're the patient sort, probably the biggest bonus of an electric foundry is effeiciency... you can dial in an exact temperature and get there without wasted energy, though it can take a few hours.
TAOW's approach was excellent and I learned a lot. I'd have been happy to copy his design exactly but unfortunately it was just too small for my personal needs. TAOW took four high temperature insulation bricks and stood these on end, forming a square void for the foundry chamber. This was a great approach and minimised the bricks needed. But I needed bigger and the next best thing for me involved a hexagonal design. This increased the volume of the foundry by almost double and made better use of the bricks depth for insulation, but it does require a lot more bricks.
I realised I'd need 18 bricks to form the main body of the foundry, this being 3 rows of 6, plus a similar row to insulate the base, making a total of 24 bricks. When ordering I purchased a nice round 30 as I know what I'm like and in hindsight I'm glad I did.
The bricks were Grade 28 Insulating Fire Bricks as these aren't cheap. At the time of purchase they were £5 each. However they're a specialised item capable of withstanding temperatures of 1500°C / 2732°F. They're regarded as lightweight though when there's 30 of them it's still a two-person lift.
Bricks are generally rectangular in shape and to achieve my hexagonal shape I needed to remove two 30° slices from each brick. The company I purchased the blocks from were happy to provide a cutting service at £5 per cut, effectively making each brick £15 each. You can imagine what I told them.
Fortunately I found a very simple way of cutting the blocks and saved myself £240. You can see my firebrick cutting guide here.
This is an indoor project and a stationary one, so having cut my bricks I set about making a sturdy wooden table. It's fixed to the wall and solid enough to take even my hefty weight. It's not strictly necessary to build a table. You could build straight off the floor. But it makes sensed when dealing with dangerous temperatures to work as comfortably as possible. So whilst this table is quite short, it's custom made to a comfortable working height for me.
Loosely arranging the bricks on the table inspired me to make the first use of my off-cuts and I decided to double the depth of my base. I drew a pencil line around the shape then arranged some of the off-cuts into a similar shape. There are lots of air gaps but air is an excellent insulator, the gaps are quite small and overall there's plenty of strength there to support upper layers.
I decided to bond these in place using ordinary expanding glue. This wouldn't survive any moderate heat, but this low down has proven to stay perfectly cool, I'm pleased to say.
With the cut-offs stuck in place, I then bonded a row of bricks on top of these. For this I used a specialised high temperature mortar specifically intended for use in a kiln where electric coils were present. I bought mine from the supplier of the bricks and it's important to get the right sort as some mortars can corrode the coils.
When it comes to mortarting I eventually found a useful technique...
Take a small amount of the mortar and add this to a separate container. Add plenty of water and mix until you have a very liquid mortar mix. This can then be painted on to the brick surfaces where thicker mortar will momentarily be applied. As the bricks are so absorbent, they suck the moisture from the mortar almost too quickly for my liking. Priming the surfaces in this way gives you slightly more working time and, for me at least, a better bond.
Once primed, take the original (unthinned) mortar and apply this very thinly to both mating surfaces. I found I could literally scrape in on and scrape it off again. Too much mortar is, I'm given to understand, a mistake.
At this stage only the outer row of bricks were laid. The centre was left hollow to allow for clamps to be used later.
I allowed the base to sit a good day to dry firmly and then I checked for flatness and level. I wanted to ensure the base was very flat and my hands told me there were imperfections. I tackled these with a combination of a rasp and some coarse sandpaper stuck to a board.
There's a fair bit of welding in this project and I'm no welder. I'm still learning and both my equipment and welding talent are on the cheap side. But it is possible, even for me, so don't be afraid to try it if you never have.
I took some 30mm (1 3/8 inch) angle iron and ran this around the perimeter of the base. With a marker pen and straight edge I was able to mark out the angles and cutting these was easy enough with an angle grinder with a nice thin blade. A chop saw or hacksaw would do the same job.
Using a wooden block to protect the soft bricks and a couple of large clamps, I was able to secure the angle iron in place whilst I welded it. I then flattened off the welds with the grinder.
As this metal frame will hold and support all of the bricks above it, it's necessary to extend it a little towards the centre. I cut sections of straight bar at 60 degree angles and welded these in place. I rushed this process and paid the price later, so I'd encourage you to take your time here and get a more consistent pattern.
With these welded and grinded back, I added another row of angle iron in the same way but sitting on top of the first. This is welded securely to the frame below it, adding considerable strength. My thoughts originally were that this frame would rise and lower directly on to the blocks, but I changed my mind as I later discovered how fragile the bricks were.
I filled in the centre void with two more bricks and mortar, making sure everything was nice and flat. It was at this point I noticed I was already starting to lose the corners of the soft bricks, so the idea of the frame dropping onto bricks had to change.
Taking some slightly smaller angle iron - 20mm (3/4 inch) - I made a frame that was exactly the same dimensions as the brickwork circumference. It fitted nicely inside the larger frame, but not too loosely.
As this frame matched the sizeof the base perimeter, I centralised it and used a marker pen to draw around the inside. This showed me how much needed to be cut away so the frame cut simply slide over, a bit lile a ring around a barrel.
It's only a small slice that needs taking off and this can be done with hand tools – a saw, rasp or even sandpaper. I actually used a router with an old straight cutting bit, though a mask is essential there as a lot of dust is created.
With the edges cut to size, the edge of the frame was scored into the base with a sharp blade. This marked area needed to be removed just a little deeper than the thickness of the angle iron.
The frame was then be mortared onto the bricks, pushed on firmly and made flush to the surface, with all the gaps carefully filled.
Whilst this was dying off for 24 hours, I returned my attention to the larger frame. I knew I needed some brackets added and I made these with more 30mm (1 3/8 inch) angle iron angle iron. I marked, drilled and cut these out, rounding the edges with my angle grinder.
I needed eight of these, measuring approximated 40mm (1.5inch) each with a 10mm (3/8 inch) hole.
Four of these are welded to the frame, each in the centre of its side. To make life easier I numbered the sides and you'll see these markings throughout for conveniece. It helps ensure everything goes where it should.
I also made eight larger brackets, these being 50mm (2 inch) long with two 6mm (1/4 inch) holes drilled in them. I new these would be separated by two lengths of inch box steel (25mm), though I was uncertain about lengths at this point.
I made use of scraps to help me position and weld these on the frame. I gave the frame a quick clean to remove any welding splatter then gave everything a good coat of rust killer.
The next day, returning to the base, the large frame lowered nicely over the metal-rimmed base. Unfortunately thanks to welding, the frame had developed a slight warp. The weight of the bricks would easily cope with this but the frame would need to be held down with straps.
But before this, the first row of bricks would need attention. If the bricks are to sit flat upon the the base, the depth and shape of the frame needs to be recessed in each individual brick. As I wasn't very careful with my framework, I needed to make individual patterns for each brick. This is easily achieved with charcoal and paper with a simple bit of charcoal rubbing.
With a router and a straight cutting bit, it's simple to recess the depth of the outer edge of the frame on each block, though this could also be done with a rasp or file.
Returning to the charcoal patterns, each is placed on a block and marked through with a sharp blade. It's worth rotating the pattern 180 degrees as this is in fact a mirror image of the frame.
With this done I glued some coarse sandpaper to a length of flat bar and used this to sand away the depth of each recess.
Whilst we're working on the blocks, it's time to recess the location of the coil and coils are big topic. So let's digress for a while and talk about resistance coils.
When considering coils, the length, type and thickness of the wire determine how much electrical resistance the wire carries. Generally, the thinner or longer the wire is, the higher the resistance. Similarly the shorter and thicker the wire is, the lower the resistance.
When electrical resistance occurs within a wire, heat is generated. If the wire is capable of handling heat for prolonged periods and the conditions are just right, it's possible to make the wire glow red hot - and that's what a resistance heating coil element is.
The Power of the coil is measured in Watts, with 1000 Watts being 1 kilowatt.
I made my coils from Kanthal wire using the coil making jig I've already shared with you. Ideally to make your own coils that meets your specific needs, you need to understand the electrical requirements and restrictions involved.
Let's start with voltage. This is likely to be the mains voltage in your part of the world. Here in the UK for example, mains voltage is 230 Volts. In the US and Canada, I believe its 120 Volts. In Japan I believe it's 100 Volts. Whatever your region, you need to know this if you're going to make a coil.
Next and to me most crucial is current. Remember it's current and not voltage that kills you so getting this right is critical. You must decide what sort of current levels you want your coil to draw.
It's VERY important that you determine the maximum current output of your outlet.
Again using the UK as an example, the average electrical outlet has a fused output of 13 Amps. As my foundry was going to be plugged into a UK outlet, I obviously needed a coil that would draw less than 13 Amps. Dropping to 12 Amps is still a little close to the mark for me, so I chose to draw only 10 Amps of power, staying 3 Amps below threshold. The closer I get to 13, the more likely I am to blow fuses and have electrical failures. The further I go from it, the weaker my foundry will be – because it's current that drives your foundry.
So you must identify the maximum current output of your outlet guys and you then drop down two or three amps for safety.
With that done you can do a little maths and see how much power you might get.
Power = Current x Volts
Potentially at 230 volts and 10 Amps, I could achieve 2300 Watts or 2.3 KW of power, which isn't too bad for a domestic appliance.
Don't be tempted to use more than one outlet to drive your foundry. This is a bad idea. Unless you've had a dedicated high current line installed, it's quite possible that you could exceed the power rating of the mains cable that supplies your outlets.
Dividing you voltage by the current (Resistance = Voltage / Current) will calculate the resistance required of your coil, in this case (230 volts / 10 amps) = 23 Ohms.
Thicker wire generally has lower resistance than thinner wire, but thicker wire is generally more durable in a foundry environment. So buy thick wire if you can but keep in mind the resistance you're trying to achieve.
Resistance wire should state the resistance rating per unit length, fr me it was 1.73 Ohms per meter. As the rate per length gets lower, the coil must get longer and if the coil gets too long it won't fit in your foundry. So keep this in mind.
I was able to calculate that I needed approximately 13 meters of wire to achieve my required resistance of 23 Ohms.
Don't forget I have a free coil calculating page on this website that can help you determine the exact length of coil you need. I also give examples of the necessary maths and formulas for those who fancy number crunching the hard way.
Rather than one coil of 13 meters, I decided to make two coils about 6.5 meters each which would be joined to make a single 13 meter coil.
The coils need to be stretched before use else and it's generally recommended that you don't stretch them more than 3 times the original coil length. This is easily achieved by gripping each end and gently pulling until the desired length is achieved.
The coils also need tails. These are easily made by unravelling a short length at each end of the coil and twisting in two more length of wire. If you're careful you can do this with a drill, but don't over-tighten or over-twist these tails as you'll snap the wires. Thickening the tails in this manner creates a better fixing point, reduces the resistance in the tails and thereby helps them to stay cooler than the coils. It's best at this stage to leave several inches of tail and the rest can be cut away later.
With the coils finished we can now get back to the bricks and in particular carving out a groove for the coils to sit in.
Now a common problem associated with electric foundries is creep. This occurs with repeated heating and cooling of the coils which can result in areas sagging free of grooves as TAOW mentions in one of his videos. This is a safety issue because if the coil comes into contact with a graphite or steel crucible, the crucible will become live. TAOW cleverly fixed this issue with stables.
Thinking on it, I decided to create circular grooves with an opening large enough to let out heat, but not the coil. I achieved this for the most part with a router. However it can be done by hand with a round file.
The bottom row of blocks require an upper groove. The top row requires a bottom groove and the centre row require both.
The blocks in position 2 don't require a full groove. This is where the coils will start and finish so leave an inch or so of block uncut. This will prevent the coil ends coming together and shorting out.
Because of the shape of the groove, there's plenty of movement room for the coil but it can't escape the groove.
Before the blocks are fitted, it's a good opportunity to install some small bolts as connection points for the electrics. I did a simple experiment with car body filler, two-part epoxy adhesive and this metal putty stuff to see how they'd respond to heat. I don't expects they'd ever get this hot, but the winner by far was this metal putty.
This is the bottom position 2 block. A hole the same diameter as the bolts is drilled to a depth of maybe ¾ inch. The dust and debris is cleared from the hole and then the putty is mixed. Very little putty is needed. This is pushed into the hole but you shouldn't be trying to fill the hole – leave some room for the bolt. As the bolt is pushed in it's turned so that the putty grips the thread. Any excess putty is firmed down or removed. Then it's just a case of waiting ten minutes. Three of these bolts are needed, two in the middle block and one in the bottom block.
A small channel needs to be made for the tails of the coils and these can be carved with a saw on the tops of the blocks only. Don't cut away too much – keep things nice and tight.
A hole is drilled all the way through the bottom block to accommodate the thermocouple. This comes with a bolt fitting and just happens to be an 8 mm thread. So I decided to carve away the opening to accept an 8 mm nut, which is easy with these soft bricks. The metal putty is used to bond the nut in place and a piece of threaded bar is used to hold things in place whilst fitting and drying. This also prevents the thread from getting clogged by the putty.
Once dried the threaded bar can be unscrewed and the thermocouple can be test fitted. You can see it's just shy of the inner wall of the foundry and this will be fine. The tip of the thermocouple is the sensitive part and that's what's most important. I've also enlarged the hole around it slightly.
And here are the bottom and middle block with their connections fitted.
Returning to the base, I smoothed down some thin polythene and taped this in place. I want to bond the bottom row of bricks in place with mortar and I didn't want them stick them to the bricks beneath them, so this bin bag provided a thin protective barrier. This is, in fact, burned away later.
The frame is placed over this and the wobble was still evident and needed sorting. A few screws were partially screwed into the table and ordinary cable ties were then looped through the brackets and around the screws. When these are pulled tight, the wobble is the frame was removed. Once the weight of the bricks was present, it wouldn't be an issue again.
I dry fitted the bottom row to ensure the grooves lined up nicely with the frame and a small amount of adjustment was necessary with a little sandpaper to fine tune the fit.
I placed a thin coat of mortar on the bottom of each block and also made sure there was some mortar in every groove. Don't overfill the grooves, but do leave enough room for the mortar to ooze out and fill the voids. Take your time with this row and get it right. It needs to be firm, flat and level to support everything that follows.
It's probably best to give everything 24 hours before thinking about the next row.
The first of the coils needs to be fitted. This is a little fiddly but with patience it can be done. It's just a matter of squeezing the tails into the channels made for them and draping the coil in place. The excess tail sticks out for now.
As the next row of bricks is added, the bricks are carefully placed over the coils, making sure not to pinch or trap them. When the full row of bricks is in place, the coil will be completely enclosed but still technically free of any fixing, allowing for movement.
Personally I bonded each brick of the the top two rows with mortar at the ENDS but I did NOT mortar the bottoms. This is important as it may be necessary to remove the coils in the future.
In truth I'm not convinced that mortar is needed form after at this stage. If your cuts are nice and true and there's no discernible corner gaps, it's probably best to dry fit only. You'll see why in a moment.
With the middle row fitted, the second coil can be inserted as before. The top row can be added and again, no mortar between the rows but if you want, you can bond the ends of the bricks with mortar.
Once the top row is fitted, if you used mortar you might want to leave things a week or so just for good measure.
The tails of the coils can now be connected to the studs. This is a little fiddly to show in situ as the position of the foundry next to the wall was quite narrow. But this image of the brocks on my bench should help explain.
Looking at the bottom block, imagine there's two tails sticking out. These are obviously connected to the bottom coil. The right tail is connected to the bottom stud. It's looped around, any excess is trimmed away and nut tightens it home. Personally I slid some high temperature sleeving over the tails first. This insulates the wire and helps retain the heat. It's cheap enough and worthwhile time investment.
If we imagine studs on the top block, these are obviously connected to the top coil. Again the right hand tail is connected to the stud below it. The two remaining left tails and connected to the one remaining stud. This in effect joins the two coils together making them one long coil.
Hopefully you can see these connections in place here, along with the heat sleeving covering the tails.
With the main body of brickwork finished, it's time to build yet another frame from angle iron which is the mirror opposite of the first large frame. Onto this I welded the left-over brackets. These correspond with the ones on the lower frame.
Some 10mm threaded bar is cut to length and this connects the top and bottom brackets with a couple of nuts. Hand tight plus a quarter turn with a spanner is tight enough. Over-tightening will only crush and crack the bricks.