Electric Kiln: the Cheaper Ever

Intro:
I recently picked up knife making as a hobby and got obsessed with it It took me a few months to get all the needed tools and materials like a belt sander, cutting tools, drill and... a furnace for heat threatment.

First I started with small butane forges built in a coffe can like those shown here: https://www.instructables.com/id/How-to-Make-a-small-Gas-Furnace/

Eventually I found out that this setup is not strong enought to perform the heat-treatment (HT) on pieces of metal bigger than a couple of inches (5cm) due to the high dispersions and little efficiency of the design.

This was a show-stopper to me, since if I could not HT a blade it would never become a proper knife.
I could not use complex/risky solutions that involved any gas, since I tend to mess-up and gas is very dangerous to handle. I tought about inducion heating but this was kind of scifi to my possibilities and capabilities.

So, afther much research and test, I made my electric kiln. I had to do a lot of research to fill the gaps of the many things i didn't  know. I found much help in ceramic/raku kiln makers since the base is quite the same.

I did not take pictures during the assembly of the project. Both because they did not seem to be that necessary, second I doubded myself that something good would ever come out if this and that it would ever work... Sorry...

WARNING:
I was cheap both in spending money and in quality, so the result, though it  works, lacks a lot of design, security and might result in being dangerous. Use all the precautions and proceed at your own risk. High voltage is used here, without much of the needed protection. Please adapt this instructions to your need and add all the safety measures that you thing might fit into it.

Also this procedure requires the use of power tools, some of those (angle grinder) are VERY dangerous and can hurt you with non reversible damage. Please take EXTREME care when using those and wear all your protective gear: safety goggles, respirator, gloves, ear protection. 

Step 1: Tools, Materials and Cost

Tools:
angle grinder
drill
1cm diameter wood dowel
pliers, screwdrivers etc.
Google Sketch-Up
metal file
pliers
papercups and stick

Materials:
16 firebricks (for this design, your might need a different number)
1 small pack (1kg) of refractory mortar
4m stell "L" bars from metal shelves
nuts and bolts
stone-wool insulation
some old computer cases (or any other sort of metal sheet)
Kanthal A1 Wire : 12 meters in my case.
metal wire.
3 (at least) ceramic whire couplers
electric wire and plug (rated 20Amps)

Cost:
20 euros for the bricks and stone-wool
25 euros for the steel bars, nuts, bolts, anglegrinder disks
20 euros for the Kanthal A1 wire and electic stuff
0 for the metal plates
10 euros for the fire refractory mortar

Step 2: The Idea

The coiled Kanthal A1 whire will be the heating element, firebricks will form the internal structure of the kiln that will keep the heat inside, a layer of stone wool will insulate the whole thing and boost efficiency. The wole thing will be wrapped into a metal case and supported by the steel bars fixed with nuts and bolts.

Step 3: Projecting the Structure

Make the project of your kiln starting from the inside chamber of it. I needed a very small, flat long chamber so I decide mine would be 6cm wide x 11cm high x 30cm deep. I was not able to find soft firebricks the "J23" type, so I used normal red heavy ones. Those are 22cm x 6cm x 11cm
I then figured out how I could arrange the bricks around the shape and size of the chamber so to combine them. During this process, since I did not want to do too much cutting and modelling to the bricks, I ended changing a bit the size of the chamber, but keeping it as close as possible to the size I wanted it.
I played around with Sketch-up and ended up with that design that I was happy with:

 - 2 bricks laying on the short (6x22) side, at the bottom forming a 6x44cm base
 - 4 bricks at both sides standing up form the 44x22cm side walls
 - 4 bricks on top (to be cut) form the 18x44cm roof
 - 1/2 brick will be used to close the back
 - another brick will serve as "door" in front.

That block of bricks packed together will measure 18cm x 28cm x 44cm The stone-wool is 4cm thick, so I needed to cut 6 sheets to envelope the bricks in. So I needed the panels of 4cm thick stone wall to be:
 52cm x 26cm = top + bottom
52cm x 28cm = sides
18cm x 28cm = front and back

The final structure (cavity+bricks+stone wool) will measure 52cm x 26cm x 36 cm

Those will also be the measures to cut the metal plates to (i decided to ignore the thickness of the plate)
Also those will be the lengths I'll cut the L bars to to form the cage that will hold it all together. I added 5/6 cm to the vertical bars to form some legs. Then, since I had some leftovers of those bars
I decided to use them to reinforce the bottom of the structure. 

Step 4: Projecting the Electrical Part

I need to heat the steel to 860/900° C in order to temper the steel I had NO idea of how to achieve this and actually it seems that there is no rule of the thumb to do it. Eventually I hoped that making a 1800 or so watts heating elements would be good enough (I was just lucky on this, should there be somebody out there that can give us all a better understanding, please, be my guest).
It's easy (Ohm laws) to find that I needed the resistance of the element to be around 45 Ohms. Given the specs of the A1 wire (resistance/length) I needed to wind up 12 meters of A1 wire in a coil and let current from the plug in it.

My coils (more in this later) are split for simplicity in left element and right element eventually connected together. each element will have 3 coils running back and forth the internal side walls.
The coils will be of a 1cm diameter and turns will be 5mm apart.
Open office Calc was my friend in this step :-) 

Step 5: Cutting the Bricks

First of all: Wear you protection gear: gloves, mask or goggles, and above all a RESPIRATOR
I cut the 4 bricks that will make the roof to length. I needed also to make the groves on the sides that will host the coils I used the angle grinder to do this. Cutting the bricks is the easy part, the groves are another story. To make the groves first mark the line with a pencil, then make two
cuts, about 1mc deep. Then with some more cuts betweens those first two open your way to a single "canyon".
This step makes A LOT of noise and of dust. I believe that my grand grand grand children will still find the dust in the lab and never get rid of it. I've then been told that wetting the bricks allows for less dust to form and to help the bricks resist the cutting/grooving operation, but I did not try so cant' tell you if it's true or not. 

Step 6: Cutting the Metal Plates:

That was also easy: given the measures I found for the plates, I drew them on a couple old PC tower cases with a sharpie, then cut them with the angle grinder. It took me three disks to cut the whole thing! Also U cut the L bars to length while I had the grinder ready.

Now you have to cut the door opening in the front plate: I put the metal plate in front of the bricks to take the exact position, market with the sharpie. Then drilled the 4 corners of the rectangular opening and joined the holes with the angle grinder. Needed to use a file and piers to get the edge of the opening dull.

Also drill two holes in the back where the electric wires will meet the resistance elements.
NOTE: those two holes are important: they need to contain the ceramic wire couplers since the ceramic will insulate the electricity flowing in the cables from the metal case.
If the wires touch somehow the case there will be a dangerous short circuit !! 

Step 7: Making the Heading Element

I decided that I would not make a serpentine with the coil of A1 wire, but I'd rather use a design I saw in a raku kiln where the coil runs horizontal along the wall, then a straight length of wire runs down vertically to form the second coil that runs backward parallel to the first.

From the spreadsheet I determined that I would need each of those coils to be made of 60 turns.
So I started rounding the (stiff) A1 wire along the dowel, counting 60 turns than leaving a few cm and repeating 3 times without cutting the wire. Did that twice to get two resistances.

Step 8: Assembling It (part1)

Now it's time to put it all together.
Start joining the L bars with the nuts and bolts, the 4 sides of the bottom, then the 4 vertical bars, and also some left overs from the cut L bras to reinforce the bottom.
You can put the last 4 top bars but let them "angled the opposite side", so that they keep parts in place but won't be an obstacle while working.

Then lay down the bottom metal plate.
The bolts will be poking inside not allowing the plate to rest "perfectly" but i didn't care... That small extra pressure will add to the hold of the whole thing.
Place the bottom stone whole sheet.
Assemble the bricks on top of the wool. Use metal wire to ensure they will stay together in place.
Add the site metal plates and put the side stone wool sheet between the metal case and the core of bricks. Note that the stone wool should not be loose but squished between the core and external plate.

At this point the hard part will be putting the coils in place. I did not make them perfect so the coil kept coming out of the grooves, so I had to use some duchtape to temporary hold them in place.
I placed one side after the other and then joined the two elements at the bottom end with a ceramic wire coupler (it has to be ceramic 'cause otherwise will not stand the heat).
Now the end I had the A1 wire ends poking out on the back side. Place the back 1/2 brick, fit the back stone wool sheet and make sure that the wire does not touch the whole directly (it will damage it) using a ceramic coupler as a sleeve for that part of the wire. Place the back metal plate, let the wires come out and insulate them from the metal plate using another ceramic wire coupler 

Step 9: Assembling It (part2)

To lock the springs in place make a bit of refractory mortar ad use the mortar to stick to the springs and the brick walls. I had to use some more duchtape to keep it there.
One you do that you'll need to ways some days (2 in my case) to allow the mortar to dry and be solid before continue the assembly
Once the mortar is hard you can remove the duchtape and check it If all is ok you can go on with the assembly.
Finish assembling it by putting the front parts, the 4 short bricks on top and final stone wool and top metal plate.
Now you can place the last 4 L bars around the top in the right way, and that will hold it all together. 

Step 10: Fire It !

Before the first fire you need to allow the mortar to rest 3 more days or it will likely break.
Check with a multimeter that the whole resistance of the A1 wire is about what you wanted it to be (check spreadsheet) Connect the wire with the ceramic couplers and plug it in

If your electric circuit does not complain (smoke, flashes or blackout) it's a good sign!

You'll feel the wire getting hot pretty soon
Let it fire for 10 minutes the first time. The springs might produce cricking sound an get glowing orange.
Allow a day rest, then repeat 20 minutes
Allow a day rest, then repeat 30 minutes
Perform a final check and you are done!

Step 11: Test

I tested my kiln but it took a lot of time (and current) to get where i needed it to be.
The kiln is closed with brick that slightly fits the opening. Some extra stone wool closes any openings that might be there.
I fired the kiln for 40 minutes, then I placed the blank of a blade I was making inside and closed it again. After another 40 minutes I opened and the blade was glowing orange as well as the springs.

I had to open the door, get the blank out with thongs, test it against a magnet and quench it in oil in under 5 seconds to be sure not to drop the temperature.

Yeppy Ya Yea!

Step 12: Future Improvements

This is not a "well done project", rather it is a proof of concept...
It works and it's fine for me but it lacks all the safety and proper performance such a device deserves to have.
Maybe one day I'll make a second one, better than this.
Future improvements will also add:
- an K type thermocouple thermometer to have the exact temperature of the blades
- an electronic temperature controller (PIC) connected to a
- a 20Amp relay that will control the power feed to the resistance.

But most of all it will have all the advices and suggestions you will give me!