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An "inclusion" is a material that is trapped within the layers of glass when it fuses. Inclusions can give a piece texture, character, and color that glass alone can't achieve.

The key criterion when using inclusions is that the foreign material, which may have different expansion characteristics than the glass, is thin and weak enough to allow the glass to expand and contract normally. If it is too thick or strong, or has a dramatically different COE, it will cause the glass to crack as it cools.

The most frequently used inclusion is probably copper, but many other substances may be used. This is an area where experimentation frequently pays off with unexpected results.


Metal inclusions

Gold foil was probably the first metal inclusion used in glass. Other precious metals such as silver, platinum, and palladium can also be used, as can less expensive metals such as copper, aluminum, and variegated red (an iridized copper carried by some sign painting supply houses).

The four precious metal inclusions mentioned above tend to maintain their color in the heat of the kiln, but that's not the case with some other metals. Aluminum, for example, tends to turn black, while variegated red turns blue (with thousands of tiny bubbles). Fine silver often turns yellow from the heat of the kiln.

Copper also changes color in the heat of the kiln, but the nature of the change is not always predictable. It can range from bluish-green to vivid reddish-orange, with dozens of shades in- between.

With many of metals, best color results are obtained when the metal is sandwiched between layers and only allowed enough air to partially oxidize.

When working with metal inclusions, it is critical to use only thin foil or leaf thickness in order to allow the glass to contract and expand normally. Thin wire can also be used if desired.

It's also possible to sprinkle metal powders, such as gold and silver, on the top surface of glass before firing. Mixing these powders with glass enamels can result in interesting decorative effects.


Mica and mother of pearl

These natural substances, which are available in powder or chip form, can be used to impart a sparkle to glass pieces. For best results they should be applied sparingly (too much causes bubbles) and covered with a layer of clear glass before fusing.

If you want to use mica flakes on the surface of the glass, first spray an overglaze (any devit spray will do), then sprinkle the mica over it. The overglaze acts as a binder to help keep the mica flakes in place.


Fiberglass strands

Just as fiberglass fabric can be used to impart a texture to the bottom of a fired piece, strands of fiberglass can be laminated between layers of glass to achieve interesting effects.

To use this technique, the resin binder in the fiberglass must be allowed to burn off. Inserting a few scraps of glass "spacers" between the top and bottom layers of glass can help this happen efficiently. As the glass heats, the resin will burn off and be totally gone by the time the top layer slumps onto the bottom. This technique also has the advantage of minimizing trapped air bubbles.

The fiberglass fabric, which generally maintains its color during the firing, can also be used with enamels or with other kinds of inclusions to create interesting patterns.


Air bubbles

Rather than being a defect to be avoided, air bubbles can be encouraged and exploited for their unusual shape and appearance. This is accomplished by deliberately trapping the bubbles between layers of glass .

The basic concept behind encouraging air bubbles to form is the creation of air pockets between the layers of glass. This can be done by sandblasting, drilling, or arranging cut pieces to promote the forming of bubbles. The tendency of bubbles to form at the intersection of ridges of glass can also be used creatively.

Textured glass can also be used to promote the formation of bubbles. Two pieces of ribbed textured glass, for example, can be aligned perpendicularly to trap small bubbles at the intersection of the ridges.

Bubbles can also be created by using chemicals or solutions that generate gas that is then trapped within the layers of the fused glass. Ordinary baking soda is the most common substance used in this fashion.

To use baking soda to make bubbles, just mix about a teaspoon of baking soda with a cup of distilled water, then brush or spray the mixture onto the top of the glass. Allow to dry, then place a layer of clear glass on top of the layer with the baking soda solution. Heat, fuse, and anneal normally. Bubbles will form randomly in an uncontrolled pattern.

Using clear glass allows the bubbles to show up best. Varying the strength of the baking soda solution can lead to different results.


Found items

Items as diverse as leaves, twigs, and cellophane wrappers, can also be used as inclusions by sandwiching them between layers of glass and using them to add unusual effects and textures to glass pieces. These inclusions will often carbonize from the heat of the kiln, sometimes leaving a fascinating "ghost image" (also called a "heat signature") behind.

For fusers with money to burn, try sandwiching a US dollar bill between two pieces of float glass and heating to 1500 degrees F. The result may surprise you.

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