Home How-To Interesting Stuff Member Art Galleries Events About Us


Making Glass Knapping Blanks Using a Microwave Kiln

By Ken Graham


How It Works

Yes, glass can be melted in a standard microwave oven. True, glass does not absorb microwave energy directly, but glass, or anything else, can be indirectly heated by placing it inside a microwave kiln, that is itself placed inside a regular microwave oven.

Referred to as a “microwave kiln”, it’s really just a “kiln container” designed to be used inside a regular microwave oven. They are made of rigid ceramic fiber with a silicone carbide inner liner. It’s the silicone carbide that absorbs the microwave energy and gets hot to over 1650 degrees.

Photo 1, kiln base and lid

The temperature reached inside the kiln depends on how long the microwave is left on. Whatever is placed inside the kiln container will absorb heat from the red hot kiln liner. That’s how glass is melted, or “fused” into a flat blank suitable for Flintknapping.

The ceramic kiln material is lightweight. It can be easily damaged, but has extremely good thermal insulation properties. At just over an inch thick, a hot kiln remains cold enough to touch when the inside is at 1650 degrees.

Photo 2, red hot kiln partially opened

How To Get Started

Buy a microwave kiln on eBay for $35 or so. Get one that ships from the US if you want it sooner rather than later.

Also get “kiln papers” on eBay. I use 1mm thick 5 inch square kiln paper, cut to size. Again, buy from a US seller or wait for months. I get mine from a seller out of Bend Oregon.

Photo 3, eBay kiln listings

Photo 4, eBay kiln paper

Get a used microwave for your shop. Free is best. Check Craigslist. Thrift stores have them for around $25. It is recommended that you get one rated for 1000 watts or higher. If you don’t have a shop, your existing kitchen microwave will work fine (if you live alone, or get permission from the kitchen owner). You will want to use the kitchen vent fan when using new kiln papers, as some smell is produced when the starch is cooked off. Used kiln paper produces no odor.

For tools, I like a small concrete trawl and a small wire brush. Also, a small steel can and metal snips or strong scissors (more on that later).

Photo 5, tools

Obtain colored glass bottles, kitchenware, glass artwork, blown glass, non-painted flower vases etc, from thrift stores and yard sales. Most colored flower vases and some candle holders are painted, either on the interior surface or exterior surface. Not good, look for scratches exposing clear glass. Blown glass is good stuff, it usually has at least one thin layer of colored glass over thick layers of clear glass. Look for the pontil remnant on the base, grinding marks and no mold lines. Wine, champagne and beer bottle glass comes in a wide choice of colors, including greens, blues, browns and black. Art glass sheets 1/8 thick can be obtained in almost any color and pattern from hobby stores, as well as vase filling flat marbles. More glass info below.

Different kinds of glass don’t always work together, experimentation is needed to test comparability. Incompatible glass will crack during cooling.

Break up bottles, vases and such into chunks and bits. Flat sheets can be cut with a glass cutter into rectangles or triangles and stacked to 1/4 inch thick or more. Curved bottle side pieces can be nested together and melted flat.

Photo 6, curved glass on base

Photo 7, flat glass stack

Glass cracking and shifting can happen when fusing together full length sheets or large bottle side sections. To avoid issues caused by shifting, add a fillet of kiln cloth scraps around the perimeter of the base platform to act as a bumper.

Photo 8, flat art glass stack with fillet

When melted, glass likes to bunch together and puddle up 1/4 inch thick with rounded edges.

Photo 9, rounded edges

Pile up glass chunks and chips up to 1/2 inch deep on the base of the kiln over a piece of kiln paper. Glass does not fuse to kiln paper but would stick to, and damage the kiln base if not covered with kiln paper.

I use a section of steel can shaped like a tear drop to contain the glass chips and chunks while stacking. I also surround the can with a fillet of used kiln paper scraps to retain the glass bits after the can is lifted off. The fillet will also allow you to make blanks thicker than 1/4 inch.

Don’t forget to remove the can. I placed a lanyard on my can to help prevent forgetfulness. A stinky mess happens the first time you leave the can in place. The finish will burn off a steel can and ruin your glass. Not sure what happens the second time. Aluminum cans will melt and burn. Don’t use aluminum cans.

Photo 10, glass pile in can with fillet around can

Photo 11, can removed

Place the kiln lid on, then place it in the microwave on high for between 15 and 20 minutes. Or move the kiln base with the glass on it, into the microwave before placing the lid on. That way you will know if the glass pile has shifted out of position during transport.

Use two hands when picking up and moving your kiln base or lid, otherwise it could break in half, especially as it cracks with age.

Photo 12, kiln age cracks

The amount of time required varies depending of several things. The output power of microwave ovens varies. The smaller countertop microwaves can be as low as 600 Watts, up to 1600 Watts for the larger ones. Different types of glass require different amounts of time, also the amount of glass makes a difference. For me, a large 4.5 inch long blank 5/16 inch thick takes no longer than 21 minutes, and not less than 14 minutes for smaller thinner blanks.

Glass will start to boil and burn if left to cook too long. Go with less time and add more if needed.

Let it cool in the microwave for two hours minimum before removing. Open it up see what you ended up with.

Photo 13, glow at vent hole

If the lid is stuck, don’t force it. It means glass has cracked and shifted during heat up. If melted glass touches the wall it will stick to the wall and fill the gap between it and the base. Simply re-cook back to fusing temperature and lift off the lid while hot. The glass will separate like taffy, hopefully without damage to the kiln. Don’t set the hot lid down anywhere that can burn. Place it upside down, then place the base on it, glass side up, until it cools.

The glass blank will adhere to the softened kiln paper. You can reuse the kiln paper several times if you carefully scrape most of the paper from the bottom of the glass with a small trawl. The paper gets thinner each usage. Eventually it will need patching from other scraps of used kiln paper. I like to use two kiln papers, one stays on the base to fully protect the surface and lasts a long time. The other can be smaller, and is reused until it can no longer be patched.

Photo 14, well used kiln paper

The glass blank will have kiln paper fibers adhering to its lower surface. Use a small wire brush to clear them off. Do the brushing outside, as it makes a lot of dust that can’t be good for your lungs.

Photo 15, glass blank with kiln paper residue

Photo 16, back side after brushing

More Info

“Water glass” (sodium silicate) can be used like glue to repair a damaged kiln. If you break your kiln in half, it can be glued with water glass and wrapped with cotton string. Coat the string with water glass and it will be stronger than original.

I have not experienced any trouble mixing different colors of modern bottle glass, not yet anyway.   Bottle glass seems to melt at a higher temperature than Art glass.

Photo 17, bottle glass mix

Not all white milk glass is the same. Molded milk glass flower vases seem to need more heat, and I have found some of them to be compatible with bottle glass. Art glass sheets, and blown milk glass are lower temperature glass and don’t fuse well with bottle glass.

Photo 18, white milk glass examples

Glass sold for fusing is relatively expensive, and will show its COE value. COE stands for Coefficient Of Expansion, and relates to melting temperature. COE 96 might be the most common. COE 96 glass is compatible with all other COE 96 glass. The other common COE valve is COE 90. Don’t mix the two and expect good results. Search eBay for COE 90 and COE 96. It’s available in 3mm (1/8 inch) thick sheets, nuggets and flat marbles of various sizes.

Photo 19, eBay COE 96 sheets, nuggets


Lower cost art glass sheets 3mm thick, are sold in craft stores and are intended for use in stained glass crafting. No COE value is shown for this kind of art glass. Actual COE is probably variable outside of 90 and 96. Experimentation will be required to determine compatibility sheet to sheet.

Craft stores also carry flat glass marbles and glass nuggets of various sizes, intended for vase filling and other craft work. No COE is shown, but they are relatively low cost. Note, some glass vase filling flat marbles have painted-on color, same as most cheap flower vases.

Photo 20, flat marbles

Be aware that color change occurs in some types of red glass. Nice transparent cranberry red dinner plate glass an flat marble glass can turn milky when fused. Sometimes not just milky red, but to milky orange, milky pink or yellow.

Photo 21, red glass flat marbles before fusing

Photo 22, red glass after fusing.

As mentioned above, most blown glass art pieces include a lot of clear glass. Colored layers are thin, but intense. The clear glass layers can make for impressive fused results.

Photo 23, red blown glass blank

Photo 24, blue blown glass blank

Photo 25, box of glass chunks

With art glass, blown glass and when mixing colored glass, many different patterns can be obtained depending on how the glass is cut, broken flaked or stacked.

Photo 26, pattern variety, same material, different stacking

Photo 27, chunks stacked on edge prior to fusing

All glass fuses to itself regardless of unknown COE and possible incompatibility with other glasses. Chunks, flakes bits and pieces from the same source can always be fused together to make knapping blanks of appropriate thickness and size.

have had some success lately using a small amount of talcum powder on the kiln paper to help keep the glass from sticking (in order to get even more use out of each piece). I have also experimented using a concave soapstone slab with some success. The soapstone is unaffected by the kiln heat, and glass that’s not boiled does not stick to it.

Photo 28, soapstone

Check my Facebook posts on Flintknapping, Art of Flintknapping, and Puget Sound Knappers group for more photos.

Photo 29, Facebook screenshot


Home    ::     How To    ::    Interesting Stuff   ::     Member Art Galleries     ::     Events     ::     About Us

©2010 J Keffer