Computer-Controlled Kiln for Slumping Telescope Mirrors

As noted by Mel Bartels, computer-controlled kilns are the enabling technology for amateur meniscus mirrors. Let’s parse that.

A meniscus mirror is simply a piece of flat, round, glass that has been formed by the careful application of heat, such that it softens enough to take the shape of whatever it sits on. And if “whatever it sits on” is a precisely made concave or convex surface, the result is a meniscus mirror that can then be fine ground, polished, and figured for astronomical use.

There are two tricks to doing this successfully. One is to create a precise form on which the glass is “slumped”. The other is to precisely control the temperature during the slumping so the glass (a) doesn’t explode, (b) doesn’t melt, and (c) doesn’t develop internal strain that could catastrophically ruin grinding or polishing efforts. The first trick will be discussed in another article. Here, we will focus on the second.

Kiln Basics

Glass slumping is performed in a kiln, which is nothing more than a very hot oven. Kilns come in all shapes and sizes, and are most commonly seen in ceramic applications . But kilns for glass work also are readily available. The main feature of commercially-available kilns, however, is that they are expensive, prohibitively so for intermittent ATM work.

Fortunately, it is easy to find used kilns really cheaply. Usually they are ceramic kilns, but that doesn’t matter as either has sufficient power to slump glass. The main drawback to getting a used kiln is that it typically does not have a programmable temperature controller, and that is a deal-breaker. Because the “careful application of heat” noted above means that the temperature must be controlled accurately, both in degrees and over time, for the glass to survive (see sidebar). And the only way to do that is to have a digital, programmable, temperature control on the kiln.

Equally fortunately, one can readily find digital kiln controllers online. Orton Ceramics would be the controller most often found on commercial kilns. But you can just as easily find something cheaper (around $100) on eBay, if you are willing to buy something made in China. And, in my experience, that works just fine.

Sidebar

When working glass in a kiln, you must be careful to not shock the glass though rapid temperature changes, or large temperature gradients across the thickness of the glass. Additionally, you must avoid inducing strain in the glass that gets “baked in”, waiting for a suitably inappropriate time to catastrophically release, or otherwise make the glass mechanically unstable during figuring or use.

There are numerous online references for key temperatures of different types of glass, but less so for safe heating profiles. For plate glass (the type of glass used in this project), the target slumping temperature is 1200F (649C), and you want to reach this over roughly 18 hours (roughly 60F/hr). More importantly, when cooling the glass, the temperature range from 900F to 600F is critical, and passing through this too quickly can leave the glass strained, and 18 hours (-17F/hr) for this is also required. Clearly the rate of heat application is critical, and that is why a digital controller is an absolute requirement.

Getting the Parts

There are four major parts you need to make a kiln suitable for glass slumping: a kiln, a controller, one or two solid state relays (SSRs), and a thermocouple. The controller, SSRs, and thermocouple are often sold together.

As noted above, getting a kiln is pretty easy if you scour eBay and Craigslist. It is mostly a matter of how much you want to spend. I was able to get two kilns for C$150: a big Paragon A88B ceramic kiln, and a second, small freebie without a lid that I took for the parts. When you look for a kiln, the main factors you need to consider are:

1. What is the maximum diameter of glass you can fit into the kiln? You should leave about an inch of space around your glass, so that will dictate how big a mirror you can make.
2. Do you have appropriate power for the kiln? Most large kilns run on 220V, and unless you want to do some electrical work to have it run at reduced power on 120V (it should still get hot enough to slump, although it will heat more slowly), you’ll need a 220V source in your home, garage, or shop.

My A88B had an inner diameter of 17”, which was ample for my target mirror of 14”, and gave me the option of going up to 16” in the future. The kiln had way more vertical height than I needed, but that does no harm and in my mind, makes for a more even heating. The price, of course, is that it takes more power to keep a larger volume hot, but in the grand scheme of things, the cost to slump glass is pretty small unless you start running a “slumping shop”.

With kiln in hand, I got a controller, SSRs, and thermocouple on eBay for about C$150. The key things to look for are a “ramp/soak” controller, a kiln-grade thermocouple, and two SSRs if you have a 220V kiln. I made the mistake of only getting one SSR initially, but for 220V you need one for each “rail” of the 220V supply. Each SSR should also come with a heat sink, to keep them from burning out. Note that it is essential you get a “ramp/soak” controller, because that allows you to precisely control the rate of heat application (ramp), and the time to hold at a specific temperature (soak). This makes programming a slumping cycle very easy (although speaking as a software engineer, the programming UI leaves a lot to be desired).

Wiring the Controller

There are two major ways you can integrate the controller with the kiln. The first way is to use the controller to switch the main power on and off per the heating profile, and simply plug the kiln into this controlled power source. A good description of this approach can be found here . The benefit of this approach is that you don’t have to fuss with any of the wiring on the kiln itself: just plug it in and go.

The second approach is to get more personal with the kiln wiring, and use the controller to switch the power flow from the main kiln power supply to the kiln elements. I took this approach because on my particular kiln, there was a Kiln Sitter installed. This mechanical device is a crude way to turn a kiln off when a certain temperature is reached. My goal was to rip the Kiln Sitter out and replace it with a digital version. The wiring inside the main controls of the kiln are a little gnarly, but readily understandable. The main power feeds the heating elements, and there are rotary switches that allow you to set the heating power of the kiln, effectively turning on or off sets of elements, or running them at half power (120V). Our digital controller merely needs to sit in between the lines going from the main control to the elements.

The wiring of the controller is relatively straight-forward. The controller will come with a wiring diagram, but here is the way I wired mine up:

Note that the outputs of the controller feed the SSR inputs, and a single rail of the 220V power goes to one output pin on the SSR. The other output pin goes to the kiln. What that does is open and close the flow across the output for each rail, turning off and on the power to the kiln. Also note that the polarity of the thermocouple is important to get right, and red is usually the positive input. If you get it backwards then increases in temperature will be read as decreases, and vice versa. I was able to easily test this by running power to the controller from a computer power supply I had lying around, which is sufficient to get the controller up and displaying temperature. I then simply used my hands to warm up the thermocouple, and verified that the measured temperature was (correctly) increasing from room temperature.

The thermocouple connections I found a bit confusing, especially which way around the ceramic insulator goes. Here is a detail of how that goes together:

In operation, I simply insert the thermocouple into the hole left by the Kiln Sitter, loose fit. In the first runs, I packed the hole with ceramic wool, but in practice I don’t think that tight a seal is important, given how hot (very!) the outside of the whole kiln is, so in later runs I felt it was sufficient to have the ceramic insulator merely cover the whole, more or less. Here is how it looks, all hooked up:

Important safety tip: don’t (as you see in the picture) have the controller box touching the kiln during operation, especially if the box is plastic. The outside of the kiln still gets VERY hot, as does the top of the kiln. I have a melted screwdriver handle to vouch for that too.

Calibration

The preceding steps are sufficient to get the kiln running. But in practice you should ensure that the controller and thermocouple are doing their job by calibrating the kiln.

To perform a calibration, you will need several “pyrometric cones“, which are ceramic devices that are placed inside a kiln and that have the property that they deform at very precise temperatures. The basic idea behind calibration, then, is to use several cones that bracket the target temperature of your kiln, and validate that the kiln is neither too hot nor too cold.

For my kiln, I picked a target temperature of 600C, which corresponds to a 021 cone. I then placed it, along with 020 (626C) and 022 (586C) cones, on a shelf in the kiln and then programmed my controller to ramp up to 600C. Note that the Orton data sheets for pyrometric cones require you to approach the target temperature at a precise rate (60C/hr). In order that I did not have to do a 10 hour calibration run, I actually ramped to 500C quickly (over 3 hours) and then approached 600C from 500C at the slower, prescribed, rate.

The following image shows the (perfect) results. Note that the target cone in the middle has melted just enough to “bow down”, while the 022 cone at top has deformed dramatically. The 020 cone, at bottom, has not deformed at all, indicating that the temperature did not approach 626C.

At this point, you now have a kiln that is capable of doing precise slumping of glass, which is the next step.