Introduction: Home Made Electric Glass Fusing Kiln
Good evening anyone interested,
My name is Dave and I am from Bristol in the UK. Before Christmas I was out shopping in a market with my daughter Ava (8 yrs old) when she spotted some fused glass jewellery on a stand. Ava thought it was fantastic and immediately wanted to know if it was possible for her to make some. Obviously, I said yes without thinking as we make lots of interesting and arty stuff together. Once home I looked into the subject and realised we were going to need some type of Kiln to fuse the glass. I looked into the cost of a kiln and to be honest found the prices quite offensive so we (Ava & me) decided the way forward was to build a glass fusing kiln from scratch.
The first issue was an enclosure - luckily this was solved by my work. They were dumping a large aluminium radio test enclosure which seemed perfect. The enclosure already had an aluminium control box on the side and was generally very robust. The enclosure didn't have a lid but was otherwise good. I did some reading around and decided early on to heat the kiln from the top as this gives the glass a better gloss finish. So I knew from day one I was going to have to fabricate a top lid with integrated heating system. To fuse glass you need a sustained temperature of about 800C and aluminium melts at about 660C - so a small problem existed. The enclosure was going to have to be very well insulated and to achieve this I used a combination of ceramic fire brick, ceramic blanket and high temperature adhesive. To fuse glass you don't just turn on the heat full power and sit back. You have to increase the temperature slowly and more importantly reduce the temperature slowly to prevent thermal shock of the glass. To control this process I used an Altec PC 410 multi-program multi-step industrial temperature controller connected to a K-type thermocouple.
I hope you enjoy the series of videos and images I have included to describe my Kiln building project. Please if anyone has any questions or comments please feel free to get in touch and I will be more than happy to try and help.
Please remember if you attempt to build any type of kiln the resulting temperatures can be extremely dangerous and could result in serious injury. Not to mention the use of mains electricity in high current applications.
Step 1: Initial Look at the Enclosure & Temperature Controller
I have link to a video here which show the enclosure before I started to line it with insulation and also a look at the Altec PC 410 industrial heating controller.
My initial though was to line the enclosure with large bricks. However, these subsequently became difficult and expensive to get hold of. I found a supplier of standard size ceramic insulation bricks in Bristol and the guy did me a great deal with all the bricks and high temperature adhesive coming to under £100 cash - bargain !
(All the bricks, fire blanket and adhesive obtained from Paul at Complete Fire Protection (CFP) Bristol)
Video of initial look at enclosure and temperature controller :
Step 2: Enclosure Now Lined With Brick & Fire Blanket ...
After getting my hands on the ceramic fire insulation brick and ceramic fire blanket I set about lining the lower half of the box. So the box is initially lines with the blanket and then subsequently lined with the brick.
In order to make as much space as possible I cut the bricks down from the stock depth of 75mm to 50mm. The bricks can be cut using a standard hand saw. Do this outside as they create a massive amount of dust while cutting and I am sure you don't want to be breathing the dust. I made a simple wooden jig (see video) to get all the brick a consistent depth.
The bricks and blanket are all secured in place using a high temperature adhesive.
Video of lines enclosure :
Step 3: Construction of the Control Box
This section again features a video showing the completed control box that will drive the heating element.
Essentially the control box consists of the Altec temperature controller driven with a K-type thermocouple. The Altec then drives a 40A solid state relay which switches power to the heating element.
I have not added too many words here because I think the video covers things :
Step 4: The Construction of the Enclosure Lid
One of the most challenging aspects of building the kiln was construction of the lid section. I decided early on I wanted the heating element in the lid as this apparently would produce the best finish for the glass.
I decided to make the lid insulation from an array of the same ceramic fire brick used to line the lower enclosure. In order to hold everything together I went to a local metal supplier and got them to make me four corner brackets plus to long and two short straight sections and also cut a thin stop plate. The brackets and straight section were all cut from 2mm mild steel and the top plate from 1mm mild steel. The corner brackets and straight sections I drilled and screwed together around the brick secured with self thread cutting screws. The resultant lid is very sturdy if not a little on the heavy side. I also fitted a handle to the front which luckily came attached to the original enclosure obtained from work.
Step 5: Choice of Heater and Mistakes Made
Ok, so now I had a fire brick lid all held together with mild steel and a handle - what I needed now was a heater. Having looked at commercial kilns I decided I needed a heater capable of delivering about 2.5KW of heat. Quite a few of kilns I had looked at used an inexpensive heating element which is essentially a coil of resistive wire which is rated at a give power at a given voltage. The element I initially chose was rated at 2.5KW at 220V AC (see the image attached).
I ordered the heater from China and it was <£5 .....
The heater looked good on arrival and I proceeded to chase out a channel in the fire brick lid with a Dremel and an abrasive tool. If you are going to chase out this ceramic brick DO NOT use a fluted metal cutting bit the brick although soft will blunt the cutting tool in about two minutes rendering useless - I found out the hard way. Use an abrasive grinding bit which works really well. The element was pinned into place using pins made from a slightly thicker version of the same resistive wire - so just about 2cm of wire with a short ninety degree bend at one end.
So I installed the element with feed wires through the lid to the top side. I plonked the lid onto the lower enclosure and connected it directly to the mains.
Sure enough the element heated up to a cherry red hot and the temperature measured by the thermocouple started to rise.
OK - two major problems became apparent :
1. No matter how much you pin the element in place it expands a great deal when it heats up. The element then sags as large hanging loops under the force of gravity. In my case the loops were hanging down some 15cm into the lower enclosure and even when the power is removed and the heater cools the heater only contracts slightly leaving the loops to flap about - it just didn't feel right to me and I wasn't happy with the heater solution.
2. The heater caused the chamber to climb rapidly in temperature until it reached about 300C where the rate of increase slowed. By about 550C the temperature was rising painfully slowly at maybe 1C/minute and was clearly not going to reach 800C in any sort of reasonable time. At this point I had taken delivery of a mains power meter. Attaching the power meter showed my heating element was actually only delivering about 1.2KW and NOT the specified 2.5KW as the supplier had stated ! I think the supplier was just selling 'an' element as various powers hoping that someone didn't check - well I did, complained and got a full refund :)
So the bottom line was I decided to ditch the first heating element and rethink the entire heating problem.
Step 6: Final Choice of Heater and Lid Construction
After ditching the first heating element I set about looking for another solution. After lots of research and surfing I picked up another bargain but this time it was a winner !
I found a supplier selling 400W 240V bar style heating elements. These still use a coil of resistive wire but contained within a high temperature glass tube and capped off with ceramic end caps with screw bolt style terminals. I picked up ten elements brand new and boxed for £13 (the lot) delivered from the UK - happy days ! After the low power issue with the first element the first thing I did was pop one of the new elements on the power meter and was happy to get a reading of ~426W.
The plan was to stack eight elements in parallel to give a total power of ~3.2KW
I used two sections of 25mm steel angle iron to support the eight elements. Steel is not a very good conductor of electricity and even worse when hot. So I faced the steel angle with 10mm strips of copper plate to act as bus bars to conduct power efficiently to the heating elements. Each bus bar is then connected up through the lid using 2.5mm copper wire terminated with a ceramic connector block.
To fuse glass you need a temperature of about 800C and copper melts at 1085C. So I figured even if the top of the enclosure was 100-150C hotter than the temperature probe as long as I didn't go any hotter than about 850C the copper would be fine - this has proved to be the case with no issues to report.
Step 7: Finished Kiln and Some Temperature Data
Without adding too many words here is a short video showing the finished Kiln :
The lid is now attached with rear hinges and the heater has been fitted. The underside of the lid has been lined with more ceramic fire blanket mainly to stop metal oxide from falling into the glass chamber during heating.
The Altec temperature controller has an automatic tuning mode which learns the characteristics of the chamber and then automatically updates the control loop PIC parameters. This worked extremely well and after calibration the temperature matches the set point to within +/-1C and even when the sequence is put into hold mode the temperature only overshoots by 1C for about ten seconds - very impressive I thought.
Step 8: Temperature Results From the Kiln
One of the first things I did was to run the kiln at full power and record the temperature at one minute intervals up to 820C and then kill the power and measure the natural fall in temperature again at one minute intervals.
I then plotted the results - this data allows me to work out the maximum rate of heating / cooling at any point on the firing sequence to subsequently optimise the design of the glass firing sequence programmed into the Altec PID controller.
Step 9: First Result From the Kiln .....
Here is the first result from the kiln .....
The pendant on the left came from the microwave kiln I had been using and the pendant on the right from the new traditional kiln which has been the subject of this article.
The right hand pendant show much better shape, colour and no trapped air bubbles :)
Step 10: Conclusion .....
I have had a lot of fun building this kiln and have learned many new skills.
I realise that there is not a huge amount of construction detail in this article so if anyone has got this far and wants to know more about any aspect of the project please feel free to contact me.
One piece of advice I will give ....... when a top loading kiln is at 820C DON'T lift the lid and shove your head in to have a look at the work inside - bad idea, just a thought !