Color in and for Glass

Compatibility - Compatibility is the measure of how well glasses of different types or colors will work together. The most obvious sign of failure of compatibility is the piece cracking or shattering into pieces; less obvious is a silvery plane inside the glass or silvered lines at the join of colors or color and clear where the glass has started to pull apart; even less obvious is strain inside a piece that causes it to crack or start to crack later in its life. This page from CR Loo has a good discussion of compatibility with figures and tests.
 

COE - Compatibility is anchored in the concept of coefficient of expansion (COE) which is a starting point only. COE is a scientific measure of how much a material expands with changes of temperature. It is measured in length per length, for example a common blowing glass is 92 x 10^-7 cm/cm expansion. This means that for each degree Celsius of temperature change for each centimeter of length, the length changes by 0.0000090 cm. For convenience, the decimal point is moved 7 places to give numbers like 87, 90 and 103 instead of 8.9, 9, and 10.3. Borosilicate glass (Pyrex) has a COE of about 30. On the same scale aluminum has a COE of 250 and iron of 120 (usually given as 25 and 12 x10^-6) COE can be measured directly over a temperature range or for glass, it can be theoretically calculated from the ingredients.

The problem with COE relative to compatibility is that the COE is typically measured at much lower temperatures than glass is at when it is going solid. COE is usually measured up to 300C (572F) while glass is setting strains as it solidifies from about 550C (1050F) down to 480C (900F)

Therefore, it is necessary to test directly for compatibility. There are two common tests

- melt small blobs of the two glasses to be tested against each other, push the blobs together and then stretch to a long thin rod/thread. When cool, break off the end pieces. If the thread is straight the glasses are compatible with each other; otherwise it will curve toward the glass with the higher COE (which shrinks more and ends shorter.) Testing is usually done against the clear being used - instead of all colors against all colors.
- cut squares of glass color or other to be tested, set them on a base glass, usually the clear, and the arrangement is fused (at about 1500F) and annealed. When cool it is observed through a polariscope and the closeness of the strain lines (or lack of them) indicates how much stress is involved.

The first test is obviously faster and more convenient for blowers.
Keep records of testing.

Color Bar - The most common way to add color to furnace glass pieces these days is to buy concentrated color bars about an inch in diameter and 15 inches long. These cost from $1-3 an inch (and are normally sold by the kilogram, not the inch) so $15 to $45 each. Different colors account for the different price, reds and oranges cost much more than blues and greens due to different chemicals and greater difficulty in making the color reliably.

Most color bar is imported and most has lead oxide in it. When used on dishes, it is covered with a thin layer of clear glass. Lead is used because it makes a glass that will be compatible with a greater range of other glasses. Incompatible color results in breakage of the pieces or thin silver looking cracks appearing inside the glass.

Color bar is used in two ways, both of which start by cutting chunks off the bar with a cold chisel or color cutter and placing these 1/2"-1" long disks in a color kiln to heat to 650-800F so they will not shatter when touched with hot glass. Normally a blackboard is placed above the kiln to write which color goes where as looking down on various light and dark disks is not easy to tell them apart. A punty with a small amount of soft glass on the end is used to pick up the disks and carry them to the glory hole for further heating and softening. The disk may also be picked up directly with the pipe.

If the color is to be applied as threads or drawings on the glassware, the soft color is shaped to a cone on the marver, heated again and touched to the glass by the gaffer, who may spin the pieces to apply threads or draw with the punty gripped in diamond shears to make shapes. Color bar may also be pulled out to make rods or stringer for use in pieces where it is picked up from a cane marver or optic. Rods can also be used with a torch to draw with the melted end like a pencil.

If the color is to become part of the body of the piece it may be applied in several ways. The most common way is to gather a bit of clear on the pipe and shape it to about the same shape as the color bar chunk. The color chunk is then picked up and heated to merge it with the clear, perhaps pushing it over the clear. Clear gathers are taken over the color and the piece blown and worked, the color forming a thin layer in the glass, with clear on the inside and out. The color in the color bar is so concentrated and light plays such tricks in glass that is is very difficult to tell how thin the layer of color is. During a visit to a studio, take a look at the crack off bucket (which may have hot pieces in it) to see how thin it is.

Some pieces have two layers of color and one way of doing this is to follow the directions above for the first color, but before the gathers, a second color is picked up with a punty, gotten very got and brought to the pipe where it is pushed on over the first color and cut off with the diamond shears, then heated again and worked down over the other color.

A trickier situation involves wanting fairly thick glass inside with a layer of color outside, usually for carving through. Here enough clear must be gathered to finish the piece and the color shaped to a disk, picked up to the pipe and worked down over the glass, which is then blown.

Colored Batch Formulas and Recipes - If a glass worker wants to do a lot of work with uniform colored glass, especially thin colors, it may be worth the trouble to color a pot of glass. The problems with mixing colored batch include changes in the color during prolonged heating, changes in the COE with adjustments in formula so it doesn't fit the base glass any more, and corrosive effects of ingredients and their fumes on pots and furnaces. With small pots, it is possible to melt colored stained glass that is compatible with the clear being used..

Chemicals for color are normally added to the powdered mixed materials of glass batch (sand, limestone, etc.) before melting. It is difficult to add colored glass to melted clear and get a smooth mix. The most common color used is cobalt oxide or cobalt carbonate for which a very small amount - 0.5% by weight - will produce a rich blue color. Henry Halem in his book, Glass Notes, spends a number of pages on chemicals and the colors they produce. On this page http://www.glassnotes.com/culletcolor.html Henry has info on coloring cullet which appears in very pruned form below. This is not an area that I am especially interested in and certainly not qualified to discuss at length.

On this page http://www.bushville.net/glasscraft/library/glass/glass_color.html  is this discussion of colors used in older batch mixes
"Among the most popular was the use of iron slag, which produced a very dark green, almost black color,
 carbon for dark green and brown glass, and clear true colors made with the addition of different metallic oxides to the mix. These are
 cobalt for blue glass,
 chromium or sulfur for yellow and green,
 nickel for purple,
 copper for gold or red, and
 tin or zinc for milk glass, which is a creamy, opaque white color.

The book, Antiques, says, "The earliest known glass beads and vessels were colored opaque blue, white, turquoise and yellow. In the latter centuries BC, green and red began to be used, indicating an awareness of the importance of furnace conditions.  Coloring agents were probably the by-products of metal working - bronze with copper and tin for turquoise while iron scale for shades of green and amber. The regular manufacture of gold-ruby and cobalt-blue glasses began in the mid-seventh century.  A compound of tin and gold called 'Purple of Cassius' was found to be good for stabilizing the color. ... [Copper-ruby will make] 'aventurine' invented by the sixteenth-century Venetian glassmakers.  Cobalt came from Saxony in the form of a powdered potash glass called 'smalt' which was a monopoly in the 18th century.  About 1830, Josef Biedel had discovered that uranium, alone or together with copper, could be used to make dichroic yellow and green glasses.  New colors became possible in the mid-nineteenth century with the discovery of chromium for lime yellows and brilliant greens.  Toward the end of the century, combining various proportions of the sulphides of cadmium and selenium."

Fragmented Color - Whatever the source of color, it may be used in several physical forms. Melted glass color is discussed above under Color Bar. Color venders add a fee for fragments, for example, Frit and Powder Pricing — add $2.00 per kilo, "00" Frit and Shard Pricing — add $5.00 per kilo, Reichenbach Cane Pricing — add $4.00 per kilo, the price reflecting the work of making the stuff. [Image at right is jars of colored frit and powder along wall of Boathouse hot shop of Dale Chihuly.  This is about one-fourth of the length.  This is extremely large holdings of color.  Just that shown in the picture is worth thousands of dollars. 2003-08-31]

• CHUNK - Chunks (1" or more) of color bar are normally not used in a recognizable form, but melted at the time of use, on a pipe. See above.
• CULLET - Broken glass from other pieces, colored or not, can be a decorative feature. I don't seem to have a strong color sense for selecting color, but I recognize nice color combinations, so I use OPC [Other People's Color] collecting fragments that I preheat and twist into shapes to make the core of paperweights.
• CANE - Long thin rods of color about the size of a pencil, pulled from color bar or supplied, which may be used with a torch to make detailed features (like a pencil) or picked up to form parallel lines of color in the piece.  Cane may also be two or more layers of glass, such as clear over color for effect or compatibility.  Cane can also be multiple colors used in building up millefiori and murrini.
• STRINGER - If color is pulled out finer than cane, to about the size of the lead in a pencil, the fine pieces of glass may be arranged in an optic or on a color marver to pick up an even pattern. Stringer may also be scattered, like pickup sticks, to roll through.
• FRIT - When color bar is broken up into bits larger than powder and smaller than chunk, the product is called frit and is available from vendors in screened sizes. It can also be made in the studio, by hammering, heating and shattering in water, or crushing in an old garbage disposal.  Frit may be laid on a marver or hot plate, including in patterns and the working glass rolled in it to pick up the bits of color which become irregular spots on the piece.
• POWDER - Simply glass ground fine and screened, it can be used by rolling hot glass in a layer of powder on the marver, by rolling in a cup, by drawing a picture on the marver to be rolled and picked up, or painted on in a carrier medium.
• SHARD, CONFETTI - If glass is blown out into a thin bubble which is shattered, the resulting fragments can be used for decoration in other pieces. Normally the shape of the edge can not be controlled and the color is very light, almost a tone. Unlike powder, which usually produces graded edges, shard can be overlaid to form well defined edges.
• PIGMENT, ENAMEL - If selected glass is very finely ground and mixed with a media, such as pine oil, it can be painted on glass and then fired at lower temperatures than will distort the piece, producing a very permanent surface. This image can be scraped or carved to produce graal (which officially is carved cased glass.) In my opinion, enamel pigment on glass is too often garish without fine control of color or shading and requires a taste for primitive art work. Thompson Enamels and Paradise Paints are sources. MF

Color Cutter

To get color bar (1" dia.)  into usable sizes, a cold chisel and hammer will work, often scattering expensive sharp pieces. A commercial cutter is a guillotine like frame with a holder for the glass that tries to keep things under control. The critical idea is a very hard sharp edge, ideally a pair exactly in line.  Several designs for homebuilt pneumatic or hydraulic have been published on web groups.

Iridized Glass

Iridized glass is unlike the other methods mentioned above in that the color is a surface effect added near the end of blowing process by applying chemicals. Iridized glass became popular in the first place because it was supposed to look like the glass that had been buried in the ground for centuries. An example is in the picture below.
  Iridescent glass is made by spray solutions of certain metal chemicals on the hot glass (in a fuming hood) or blowing powders of same (also fumed) or applying a chemical compound and then reducing the metal compounds to a thin coat of metal.  Reducing is done by cutting off the air supply to the gloryhole (or adding gas to the air flow after the regulator) to produce a gas rich hot flame (yellow) that steals the non-metal from the compound.  Depending on how the material is applied, the result is a thin vari-colored film or a silvery sheen with color tones, like the vase in lower left of the picture.   Tin chloride in dilute hydrochloric acid is one of the (relatively) safer spray compounds.  Silver nitrate (which kills and stains skin) is one of the applied compounds.  I believe Henry Halem covers this in his Glass Notes (www.glassnotes.com ) and Art Allison (www.allisonglassworks.com ) uses the effects against very dark glass - examples abound on his site. 2002-02-09

Striking Color

Colors which contain gold (gold chloride, gold ruby) and silver and some other metals will strike - release the metal to produce a sheen or an internal color effect.  This is different than the iridizing mentioned above because that is a thin film applied externally, while the striking colors are thicker and may be internal or external..

Heat Sensitive Color

There are certain glass mixtures which change color depending on their heating history. The most widely known use of these is in so-called Burmese glass of the late 19th century. The glass, when first cooled is an even yellow but when reheated to just above the annealing point turns a rosy pink (like a Burmese sunset per Queen Victoria.) I do not have specific information on glasses which do this. The picture at right is mine of glass in student center museum at Texas A&M University. 2002-02-09

PhotoSensitive Glass

Corning developed several varieties of photosensitive glass, one of which is available from Color Rods, in Seattle.  Photosensitive glass will take an exposure through a negative or through markings made on the surface of the glass.  The exposure is done with UV light and the development is done with heat, so in a way it is like the glass above.  The color, as seen in samples at the 2003 GAS demonstration, is a pale lavender - mild purple.  The glass must be worked, annealed and reheated under as dark conditions as possible and the exposure is long. 2003-08-31

Glowing Color

Glass can glow when struck by UV light, such as uranium glass.  Spruce Pine Batch sells color bar that offers a variety of colors.   Glass can also glow when chemicals absorb light and later give it back - phosphorescence - available from Glow Glass LLC Products  http://www.glowglass.com/pages/products.html  and used nicely here Corning Feb 2006 People the incalmo lamps partway down the side. 2006-04-01

Sources of Color -

Bullseye - makes glass primarily for fusing, although they are getting into blowing. Their glass is nominally COE 90, which should work with Spruce Pine, but they discount that and urge testing of any kind of color. They market their color as tested compatible, which means they test against a specific clear base at fusing temps (which are higher than the COE temp.)

East Bay Batch (not a very good link) sells Gaffer Colorbar from New Zealand

Olympic Color Bars sells Kugler Colors®*, Gaffer, Zimmermann, and Reichenbach (also known as Q-Color). Link to HTML e-mail message/newsletter with color images of Gaffer

Reichenbach - brand made in Europe, sold through distributors in US

Zimmerman "is lovely : stuff, but costs more."

Fredrick and Schreibler "is garbage.. has been for ..."

Weisethal

Flying Colors in New Mexico sells bar through Spruce Pine Batch, only sells pot quantities direct. 2005-02-21

SAMPLES OF COLOR

In order to illustrate the large number of color choices, below is a table borrowed from a sale announcement on Olympic Color Bar (see copyright notice below, used under fair use copyright law-link) In this case Olympic has made their part numbers from the Kugler numbers, adding R for rod, P for powder, and a size indicator for frit. A (T) after the name indicates it is transparent.

Most glass workers examine samples of colors and a select a few (dozen?) to memorize and keep in stock. It is possible to buy a "book" of color chips in clear plastic pockets to keep track of the colors.  The internet has made access to sample colors much easier, subject to the quality and adjustment of the monitor.  It is certainly a good idea to have some real samples to compare to the same color on the screen to see how they vary and thus help guess how the others vary.
Most users write the numbers on their color bar with marker because many of the colors are intense and dark so a bar of red may look black. Discussing color with an artist is often a bewildering array of numbers that make sense only if you know the brand. (e.g. "I tried the Kugler 27 but I really didn't like it and the 32 was darker than I wanted ....")

KUGLER Colors

REICHENBACH Colors

Table contents copyright © 2000 Olympic Color Rods, Seattle WA, unless otherwise noted. All rights reserved.