heated glass

project.

i think you're just trying to heat up the glass to 150F so that the wax will bend, and not trying to bend the glass itself?

glass doesn't like to have any part of it more than about 4 degrees difference from any one place to another. you need to heat it very evenly to accomplish this. think copier machines, which use light, or maybe a water bath. you'd have to use a lot of strips to make it be evenly heated.

why not use a crock pot...they get well over 150 at medium set. or get one with temp controls.

I think the heat of your problem is "The size if 20" X 24" and the glass could be from

1/4" - 1/2" thick and is supported fully from the underneath side." The glass should NOT be supported from the underside, which will keep it cool and stressed across it's thickness. I would recommend building a box (insulated somewhat) that holds the glass at the rim, that is about 6" deep. In the box or at the bottom of the box, put one of the following: a griddle, a hot plate, a wire heating element. The advantage of the first is that it includes a heater control, but maybe not that low. The dial would have to be protected from the heat. With the latter two, you would need either a controller or try manual control with a dimmer. The goal is to evenly heat the glass from below until it gets to the desired temp and hold it. If you have a big enough kitchen oven, you could put the glass in the oven and put it on warm, checking the temp with an oven or candy thermometer until it steadies. I do this with plex and casting wax in a gas oven with a long warm section and a low numbered reading of 200F I have built boxes like the one above from sheetmetal and frax and from insulating castable, but if you limit the temp to below 200F and the heat source is well controlled, you can make the box of wood.

Why so thick?

The thicker the glass the more temprimental it becomes with heat. You would be better off with as thin of sheet as possible.

Use Pyrex sheet glass. No window glass will hold up to thermal shock. A heat gun won't phase Pyrex. Unless you stick the nose to one spot only and don't move it. Pyrex's strain point is about 986F.

Randy Hansen SC Glass Tech. Scam Diego, Comi-fornia

Yes, but will the thin glass hold enough heat to allow the wax to lay flat, etc.

Snip

I guess I should have laid out the whole process to be more clear.

I put a mixture of heated wax and pigment on the heated platen. I use a heavy glass muller to combine the wax and pigment. The muller looks like a large heavy paper weight with a vertical handle and a flat ground bottom. You move the muller through the wax / pigment mixture in a figure 8 motion until the pigment is thoroughly mixed into the wax. Ever so often, you need to restore the tooth of the platen surface by grinding it using the muller and a mixture of water and 400 grit Silicon Carbide. Unless you can keep the wax mixture right at the melting point about 150-155 F., this process is a bitch. I chose the heavy glass because of the weight of the muller and the possibility of mechanical abuse when the wax sets up from time to time.

Would Pyrex sitting directly on a large hot plate work ?? If so, what thickness would be the best ??

Thanks for suggestions, Ed Angell

I would have suggested a hot plate or warming tray if I thought you could get one as big as you want. And with what you are pushing on, I think you need 1/2" or 3/8" glass, not window glass I still think you will be much better off with a box to buffer the local effects of whatever heating source you use, which could be a small warming tray or a hot plate. I think that putting Pyrex directly on a hot plate will create a wobbly work surface and depending on how you control the temp, may still get too hot locally, even for Pyrex. If you moderate the voltage, say with a dimmer or variac, it might work temp wise but still be wobley. To be honest, you might be able, with a wooden box lined with thin fiberglass insulation like used in ceiling panels, to get up to 150 with a hundred watt light bulb or two and use a cheap light dimmer to control. Since you are well below the boiling point of water, you could put a baking pan in the box and pour boiling water in from a teapot but getting it out and keeping the temp near 150 might be a drag (he said having softened pitch for repousse)

Perhaps you could try making a covered stand and "bowl" for the glass plate. Make it open underneath and paint the backside of the glass black with enamel paint, then suspend a 600W halogen shop-light about a foot under it, pointing up at the black surface. This will get very hot (and heat evenly over the full surface.) The lamp will need ventilation though. If yer into electronics, there are several methods for controlling the temperature accurately.

-- At it's heart, all a fractal is is a simple unbalanced dynamic, like cos(1/z)*z. A fractal is unbalanced in such a way that it can never be solved, and the longer it is "computed", the more complex it becomes. It is literally a twist or gotcha, on infinity.

Here's a thought...

Yesterday I just purchaced a small desktop ceramic heater for my office. They have two settings, 1500 watt and 750 watt. Plus they hay have thermostate to control the temp. It cost me $30.00. Putting my hand by the face of the heater I can tell it is much hotter than 150 degrees.

A constant flow of hot air on the glass would do it without any danger of direct contact of the heater on the glass. I'm sure you could dial in the right temp range with a little trial and error.

Using Mike's suggestion to coral and contain the heat would make a big difference.

After your expanded explanation, I agree you need a thicker plate. Just keep in mind that glass is a poor conductor of heat. It will take a while for the heat to saturate a thicker mass of glass. But in time it will do it.

Just thought that might work for you.

Randy Hansen SC Glass Tech. Scam Diego, Comi-fornia

I would think so as long as he has a constant heat source. If the heat source is removed, the glass will cool quickly.

Randy Hansen SC Glass Tech. Scam Diego, Comi-fornia

"600 Watt halogen shop light a foot below" will produce several hundred degrees at this distance from a black surface. we are only talking about needing 150F folks.

600W is only applied initially to decrease the warmup time. Electronic PID algorithm modulates the light intensity to any preset value.

-- "Perhaps it isn't that people are intentionally redefining standards, just that our logic is changing faster than our technology." Mark Jones / Helios Studios 20050109

halogen lights don't like to be switched on/off a lot, and they don't like dimmers on them either. both lead to premature aging of the bulb and quick burnout.

Actually, high voltage halogen, like shop lights, don't have the problems with dimmers that low voltage decorating lights do and almost any dimmer with sufficient rating (since many shop lights are 1000w and most dimmers are 600w and shouldn't be used at full rating without cooling) will work. However, 600 watts is still too much and a couple of hundred watts at most will do the temp needed with a cheap household dimmer, not a $100-200 PID controller.

I'm not fully sure about this. I thought the halogen concept worked on the basis of needing that high temperature for sustained operation. In any case, a design where the light remained "mostly on" rather than "nearly off" would probably be better for it. That design might best using two lamps - one large one for the fast initial heating, and a smaller one to maintain a set temperature.

600W was only a guesstimate. I assume, with a chunk of glass a half inch thick and a foot square with a few pounds of wax on top of it, it would still take at least 15 minutes to warm at full power. That's a lot of thermal mass to heat. Of course this too is speculation; I've not tried it. Nor do I fully understand exactly what the OP is trying to accomplish.

I've used a 500w halogen light before as a heat source and must say, it does make a good deal of thermal and radiant heat. However, even though the glass plate would be painted black, the coefficent of conduction would not be all that high. The goal would be to modulate a halogen in the last 10-20% of "fully on" for the desired temperature. Increasing the surface area of the bottom of the glass and/or using a higher-conductivity coating will obviously help thermal transfer efficiency and allow an even smaller lamp to be used. I recall some black paints or powders used in telescope design which boasted like

98% absorption or so. That would work nicely. :) They do make 250W halogen lamps for indoor lighting fixtures, and of course the lamp could be moved further from the glass and opened to more airflow as to reduce the end temperature. But then the warmup time is increased. Perhaps the best method may be one lamp on a movable track underneath the glass. Put it close to get it the glass hot fast, then move it a set distance away to keep that temperature. Again, "fast" is a relative term (especially with uneven heating of glass!)

As for PID, the average weekend warrior might hop on over to sci.electronics.design and ask about PID alternatives. It's not too difficult anymore for someone adept in electronics to implement PID using today's jellybean microcontrollers. The cheapest PID solution for a non-electronics guru might be to have someone there build a controller for them. The parts wouldn't be too expensive, but programming/labor might put things back in the same ballpark (unless someone has already made a PID controller for a similar use.) But there are other solutions also. A simple (precision) linear temperature regulator might work just as well. A one-off design could probably be engineered, built, and shipped for under a hundred bucks, but I assume someone will have already made something similar. And like you said, a household dimmer (like a wall-mounted dimmer switch or a router speed control) could also work, provided the lamp is okay at reduced temperatures. If the wax needs to remain a precise temperature though, a dimmer might give you fits. I suppose the temperature would stabilize at some point, but it would be difficult to "fine tune" it without a lot of fiddling. Perhaps with experience this would become predictable and usable. Maybe try the "distance" method first, then if that doesn't work try the dimmer method, and if that doesn't work, upgrade to a linear or PID controller?

Regards, Mark

high voltage halogen? shop floods and torchere lights both use 120v, and use virtually identical bulbs. you can dim them a bit, but too much burns them out pretty quickly.

somewhere i saw a link to someone who used one of them in a small refractory lined box to make a kiln which was capable of melting glass.

regards charlie cave creek, az

MR11 and MR16 lamps are small halogen floods that run of 12 volts and are widely used for decorative and specialty lighting. Under counter lightstrips that have two, three or four hockey puck shaped lights come in two version, one of which takes 120 volt bulbs the other takes 12 volt bulbs. All the the 12 volt require a special dimmer to deal with the power supply, commonly a small switching power supply these days. Quartz-Halogen and metal-halide bulbs remain bright by a complex chemical reaction that takes the metal boiled off the filament that would normally condense on the "glass" tube and reunites it with the filament at the hottest spots, which are the thinnest and most likely to break. The glass tube is quartz to stand the heat. Dimming them is possible-most theater lighting is now halogen-although I would not be surprised to hear that life is shortened.

I agree with this, and for that matter, I think he would get by just fine with an ordinary 100-watt incandescent bulb. They get plenty hot, and it only takes them a few minutes to heat up. If I were in charge of this project, , using the combined input of others here I would take a wooden fruit crate and line it with tin foil. Then I would take a $6 clip-on utility light, remove the aluminum reflector, equip it with a

100-watt l> Actually, high voltage halogen, like shop lights, don't have the problems