Make:

most glass "pebbels" you can buy are not melted into pebbles, they are tumbled in a rock tumbler type setup to round off all sharp edges and the longer they are ran trough, the rounder and more pebblish they get.....

I am not sure if you can actually get it hot enough in there to "Melt" the glass, but probably more of softening it. The microwave might only get to around 800 degrees.... glass needs atleast 2700 to 4200 degrees celcius to melt depending on the silica content and if it is soda lime glass.

Thanks for the response.

I know how the glass pebbles are made, but I don't have glass that is suited to that method. Basically I want to smash old jars and bottles, melt them and mould them.

I have seen a technique where the glass is heated to its melting point using a blowtorch and then placed in a microwave. It then continues to melt.

What I am looking for is an idea of of what material I could keep the glass in, while it is being microwaved. I have seen microwave kilns for sale but cannot figure out what they are made of.

Graphite

A lot of it depends on the type of glass you're interested in working with. Note that there's no such single thing as "glass", but, rather a whole variety of different materials that are classified as "glass". Some of the glass materials melt at reasonably low temperatures, while others melt at some fairly high temperatures (I say melt, but, as everyone who has worked with hot glass knows, it actually just softens rather than melting.).

Anyway, you'll need something around 1200-1400F to melt some of the glasses, although some of them will soften down around 900F or so (Look up the terms "slumping" and "fusing".). Some hot glass shops use special kilns for this, and these usually have a ceramic liner of some type. There is a special, high temperature "paper" which can be used as a liner to prevent the glass from bonding with the ceramic liner/base. There are also other molds/bases/substrates/etc., which can be used to prevent the glass from sticking.

Also, note that if you're contemplating fusing different pieces of glass together to make an artwork project, you'll need to match the coefficient of expansion of the various glasses; otherwise, the differing coefficients of expansion will cause the finished piece to shatter when it cools. Even for a single piece, you may need to run the piece at an annealing temperature (800F or so) for many hours to allow the thermally induced stresses to relieve themselves before cooling the glass to room temperature. Otherwise, the finished piece may contain stresses which will cause it to shatter unpredictably.

There are some methods in place to analyze a piece of glass for stress, usually involving polarized light and a crossed polarizer.

Oh, also, when glass is heated to a high temperature, it may lose it's normal electrical insulating properties, and become conductive. However, I don't know that I'd recommend trying to melt it in a microwave, even if the electrical conductivity would allow it to absorb microwave energy, since that has no provision for annealing it as it cools.

Look up "slumping", "stained glass fusing", and "annealing (glass)",

Does that explain things a bit?

Dave

Make had an article on melting metal in a microwave with a silicone carbide sharpening stone; an artistic maker trying to build a toaster from scratch smelted iron in his kitchen. If that didn't end disastrously, glass may be worth a shot if you have an oven lying around.
What you can line the oven with is firebrick. Know that there are 2 main types; hard and soft. Hard you may find in a hardware store, its often used as a base for oxy-acet. welding as well as for fireplaces. It doesn't insulate terribly well, but it will absorb heat until it's glowing red hot at almost 2000 degrees. It won't melt, but that may end poorly for your microwave.
Soft firebricks are specialty ceramic which is so soft it can be carved with a spoon. It will protect your microwave while reflecting radiant heat back onto your cupola or whatever you're melting the glass in (you could hollow out a firebrick for the job).
You said you don't care what the finished product looks like, which is probably good for all the reasons listed above. You may get a nice paperweight or two out of it though. Christmas gifts?

One difference, though, is that metals that have been melted and cooled don't spontaneously explode due to stresses built up by the thermal processing. Glass does. It's not nice to leave little random timed hand grenades laying around. The chance of injury is, of course, low, but I certainly wouldn't want to take a shard of shattered glass to the eye.

The ceramic fire bricks are truly amazing. Those are related to the thermal insulation used on the Space Shuttle. I've personally seen someone heat one of those things up with a Propane torch until it was glowing red hot. Then, in the time it took him to set the torch down on the lab bench, he was able to grab the tile with his bare hand and not sustain a burn! Very amazing.

The problem, though, is that those ceramics aren't necessarily easy to find, and are somewhat delicate, due to their softness.

Dave

For help with construction of a Parabolic Dish Solar Micro-Foundry, call 480 528 0632. It'll melt glass for sure, most common metals and who knows what else besides. I just located a supplier for special-purpose 90 and 95% solar reflective metal. If you're serious about solar, let me know ....

I don't know anything about the microwave method, but it sounds a bit wacky (and dangerous) to me.

If you want to build a -real- kiln or furnace, you can start here...

http://www.duralite.com/store/scripts/prodlist.asp

John

There are several microwave kilns on the market. User experience is varied. The manufacturers want you to very carefully stick to a single glass identified by COE - coefficient of expansion. I will be experimenting with a medium size one this weekend, loaned to me by the local City Arts glass department. Decorative arts glass is usually COE 90, 96, or higher. Pyrex is in the 30's.

Glass will reach the fuse stage at between 1350 and 1500 degrees Fahrenheit, where bits become one piece, but colors do not mix particularly. 'Virgin decorative glass fully melts at around 2200 degrees -- call your local glassblowers to confirm!
The low-end can produce a slumping and a 'tack-fuse' where the pieces remain independently intact, but are heat-bound together at the interface.

I have pot-melted bottle-glass in my ceramic kiln, but half-inch tempered shelf glass has to get hotter to deform substantially.

Household waste glass bottles are generically in the COE 83-87 range (according to NIST), and if you crush fairly finely and mold them shallowly, it should work nicely --but you must carefully monitor that microwave technique. Most suggest testing at 30 second intervals. The reason for the torch startup is to prevent splintering as it heats, so for small 'frit' as opposed to 'cullet' you don't really need the torch. Just really watch the cool-down. Too fast and the glass does not anneal, causing shock-cooling crazing and cracking. This is actually a very complex subject but a tremendous amount to info is online.

Now I just gotta comment on the remarks about 'glass pebbles.' Firstly, I crush and tumble glass in my cement mixer --it turns out most pretty flat (bottle glass is usually less than a quarter inch thick in the walls) and with a beautiful frosted 'sea glass' surface. If the 'pebbles' are shiny transparent pieces commercially referred to as 'half marbles,' then you want to know that they ARE often formed from recycled bottle glass that is melted in a crucible then dropped onto a moving stainless plate --probably should say 'dripped.' I'm just sad this is not being done in the US; we generate the waste, it ships overseas for processing, then back to us for decor. Ugh!

Another suggestion: when you realize that physically glass is a super-cooled liquid, you will see the logic in the fact that glass is also self-healing --over time the sharp edges round out themselves in a limited way. This 'aged' crushed glass is what is packaged for use in walkways and ground cover, and those trickling waterways in the upscale yards. By NOT being tumbled, it just sparkles... although the tumbled glass shines when wetted.

The mfgrs of the microwave kilns claim to have something lining the inside of the tiny ceramic kilns which 'focuses' the microwaves... I'm researching that now... hum-m-m --could be a marketing ploy. The 2inch ID kilns are about $100 new and the 4 to 4-1/2 inch ones seem to be running between $130 - 150 USD. See both from 3 or 4 manufacturers on eBay and at Amazon. BUT these are for fusing, NOT melting. Truly melting will require a different container. I melted in a terracotta planter and it dripped from the hole in the bottom to make lovely drizzle-stringers and puddled on a kiln-washed shelf below. Cute stuff, and not too fragile but looks very delicate.

Sorry for the epistle, but this is my current project, and wanted to share what I've learned over the last 3 years. You can see some of my waste glass stuff on Etsy in the eagleturtle shop. It sells fairly well at craft fairs just because of uniqueness.

Wow. That was awesome. I for one really appreciate your post. I learned a lot and it sparked a lot of ideas.