WHAT IS GLASS?

THERE ARE MANY DEFINITIONS:

Glass is a state of matter…

Since the glass we will use is primarily formed from silica we could also define glass as:

a siliceous supercooled liquid……

Siliceous refers to silica which is usually the main ingredient in glass. Glass in its basic form has one atom of silica (red colored balls on the left molecule) and two atoms of oxygen (white balls). There are many other complex chemical variations of glass.

Glass is made of three basic

components:

FORMERSare the basic ingredients. Any chemical compound that can be melted and cooled into a glass is a FORMER.

FLUXEShelp FORMERS to melt at lower temperatures.

STABILIZERScombine with FORMERS and FLUXES to keep the finished glass from dissolving, crumbling, or falling apart.

Silica. Other FORMERS include: - Anhydrous Boric Acid - Anhydrous Phosphoric Acid but melting sand by itself is too expensive because of the high temperatures

required (3360°F) THEREFORE... We add fluxes such as:-Soda Ash-Potash or- Lithium carbonateBut FLUXES also make the glass chemically unstable, liable to dissolve in water or form unwanted crystals. So we add stabilizers such as:-Limestone · Alumina · Magnesia · Barium Carbonate · Strontium Carbonate · Zinc Oxide · Zirconia

A supercooled liquid is a liquid that is too cold to freeze. The molecule on the top left shows the random arrangement of glass atoms.

Silica in its solid form has the organized, rigid structure shown below. Quartz (below right) is an example of "solid" (crystalline) silica.

If silica goes from the glassy to the solid state of matter we call that devitrification- this can occur if we are not careful with the temperatures we expose glass to in a kiln……

Glassy state of matter (above).Solid state of matter (below) and quartz crystal (inset below).

You may ask "If glass is a liquid why doesn't it move?"

It does but very, very slowly. The graph at the left shows that it would take glass, at room temperature, over thirty million years to move the distance it would in one second at 1800 degrees Centigrade.

Glass exists naturally in our environment. It can take the form of obsidian from volcanic eruptions.

obsidian

Glass is also found in extraterrestrial sources such as basaltic moon rock and tektites which probably originated from outer space.

basaltic moon rock

tektites

Medieval glass (the formulation on the far right) contained over 50% lime and potash relative to the amount of silica. This meant medieval glass melted at lower temperatures (so they needed less heat and less fuel!) but it also corrodes much faster particularly into today's industrial atmosphere.

As stated before you can make glass from pure silica but it would take temperatures over 3000 degrees F. to melt. A flux such as soda ash is added to lower the melting temperature. Limestone is usually added as a stabilizer. The mixture is then heated to over 2700 degrees F.

The graph on the left shows different glass compositions.

Light & Glass

How light reacts with glass, will ultimately influence what we see……Light can be either reflected, transmitted, absorbed or some combination of the three as it encounters a piece of glass….

The color we see will be the interaction of the spectrum of light coming to the glass and its interaction with the chemical composition and physical structure of the glass…

In this example a piece of glass that appears white is reflecting all the colors of the rainbow back to your eye.

In this example a piece of glass appears CLEAR because all colors (all light) are transmitted through the glass…to get glass to transmit all light (i.e. to be clear) you must add a decolorant to the glass mixture as you make it. Antimony is one such decolorant.

Light & GlassLight & Glass(continued)(continued)

There are a few basic ways to see color from a piece of glass. Glass is colored by

(1) impurities in the batch ingredients, or (2) by one of four processes: a. using a dissolved metallic oxide to impart a color throughout (see opposite pane) b. forming a dispersion of some substance in a colloidal state. c. suspending particles of pigments to form opaque (reflected) colors d. diffract reflected light from the surface (usually causing a rainbow effect)….

In this example iron oxide is dissolved in the glass during it manufacture. As white light goes through the iron oxide only allows blue and yellow light to pass (red is absorbed), thus you get a green (blue + yellow) transparent glass!

Light & GlassLight & Glass(continued)(continued)

On the left is an example of forming a colloidal dispersion to produce an orange color.The medieval red color was produced using gold in a colloidal suspension BUT it was a bit more complicated than throwing a gold chain in a hot batch of glass! That would have produced a red color so dense as to be nearly opaque. So thin layers of red were ‘flashed’ throughout a clear glass matrix in a time consuming process that resulted in an amazing red glass seldom duplicated today……

In this method a chemical is suspended rather than dissolved in the glass during it manufacture. Also known as colloidal glass the submicroscopic particles filter out or scatter certain colors. Here cadmium selenium particles only allow red and yellow light through to give you orange (yellow + red) glass.

Light & GlassLight & Glass(continued)(continued)

On the left is an example of a sparkling effect gained by the rough or prisimed surface of the glass that diffracts and scatters the light in odd directions as it passes through…..

This glass appears flawed or sparkles because light is not exiting at the same angle entered at. Note that all colors in the spectrum are scattered so the color is white or clear..

THE END