Transparent Colored Glass Samples – Part 1
Around 1858, iridized glass was rediscovered by Hungarian Ludwig Lobmyer when smoke from a fireworks celebration accidentally drifted into his glass ovens. He soon determined which chemicals produced the various iridescent colors. He exhibited iridized glass in the Vienna Exhibition in 1873, but it wasn’t until 1889 that European iridescent glass attracted the attention of Louis C. Tiffany, who’s “Favrille” iridescent art glass was a process of mixing chemicals into molten glass and developing the color by various heating methods. The process required a high level of skill and was very expensive.
It has been stated that Harry Northwood wasn’t interested in the Tiffany-type glass until he heard that it was popular in New York, then he stated that: “he could made stuff like that and sell it cheaper” … and he and several other companies did just that. The iridized glass was advertised under various names; but later, became known as “Carnival Glass”. This process required the molten pressed glass to be sprayed or dipped in a metallic salt solution and re-fired in a reducing (smoky) flame to develop the color. Additional colors could be added to the glass surface by re-spraying and re-firing. The actual process was never patented or published. Although Fenton was the first company to market carnival glass, Thomas Dugan was known to have experimented with iridizing techniques as early as 1902. With a few exceptions, the color of carnival glass is stated in terms of the base color of the glass rather than the color of the iridized surface.
Copper (blue) and manganese (purple), some of the oldest chemicals used in coloring glass, have been found in Egyptian glass of about 1400 B.C. Cobalt blue was used in enamels in the Ming Dynasty (1368-1644). Gold-Ruby Red glass was developed in 1685, but the technique was lost until it was re-discovered in the middle nineteenth century. Uranium (yellow) was first used in 1789 and chrome (green) in 1795, and a patent for selenium Rose Red glass was issued in 1894. The colorants (chemicals) discussed here are from some of the working formulas used commercially by Thomas E.A. Dugan and Harry Bastow.
Glass making requires three basic types of materials: glass sand (silica), which gives glass its form when cooled; alkali (soda ash or potash) to lower the melting temperature of sand to more practical levels; and lime to stabilize the sand and alkali mixture to make glass uniform and durable, but glass rarely contains only three ingredients.
Making colored glass is a complex chemical process that involves dissolving or dispersing colorants (colored chemicals and/or metals) in molten glass to produce a specific color. The intensity of the color depends on the quantity of the colorant used, composition and thickness of the glass. Some colorants can even produce a range of colors depending on the amount oxygen consumed at melting; the temperature of the glass during melting, cooling, re-heating and annealing. Color is also determined by the type and amount of alkali and other minerals used; as well as, impurities in the sand and ingredients used to make glass. No wonder there are so many variations in base colors of carnival glass.
The glass shards recovered from the Indiana, Pennsylvania glass dump were discarded by the following glass companies from 1892 to 1931: Indiana Glass Company 1892, Northwood Glass Company in 1896, National Glass Company 1900, Dugan Glass Company 1904 and the Diamond Glass Company in 1913; and later, changed to the Diamond Glass Ware Company. After cleaning, the shards were examined under black light (UV long wave) and 5000 ° K white light; sorted according to their base color (blue, amethyst, etc.); and then resorted by its intensity and type of glass (transparent, opaque, etc.). None of the molten glass samples were uniform in thickness, so the intensity or shades shown are approximate gradations of each color. Shards with pressed patterns were separated for identification and are discussed in a later article. The following will cover colored glass produced by these five glass companies.
Clear Glass - Two kinds of clear glass were found.
Manganese was added to molten glass to neutralize the green color imparted by impurities such as iron. Later, selenium and cobalt (blue) or nickel oxide was substituted to eliminate the color change (purple) in the glass caused by sunlight reaction with the manganese. Lead, boric acid, arsenic, etc, were also added to condition and stabilize the glass. Some, but not all, of the recovered glass samples exhibited a weak yellow to peach/orange fluorescence (1-B) when examined under a black light. Some white and marigold carnival glass exhibit this type of fluorescence which is due to the manganese that was used in the early manufacture of clear glass.
The clear glass was used in the manufacture of marigold carnival glass and is one of the “exceptions” previously mentioned. As soon as the molten glass was pressed with a pattern and removed from the mold, a ferric chloride solution was sprayed on to the molten glass and re-fired to develop the color marigold. According to one reference, a lead based glass is considered best for iridizing. The quality of the iridescence varies between Dugan and the Diamond Glass Company.
In the Windflower patterns, produced by both companies, the iridescence ranges from almost nonexistent (2-A), to average iridescence (2-B), to radium iridescence (2C), to dark red-orange “pumpkin” (2-D). Even though 85% of Dugan/Diamond carnival glass patterns are found in marigold which is the most common color; some patterns are difficult to find in marigold. Occasionally, marigold iridescent is also found in some patterns with a base color of aqua (2-E), amber, lime green, olive green, yellowish-green and pink.
Amethyst - Only four shades of amethyst were found.
Amethyst colored glass was made using red iron oxide and manganese oxide. The density of the color could be changed by amount of ingredients used. Black glass (3-A) was made using large amounts of colorants which blocks all light and appears black. Chipping a thin sliver of glass from the edge of a black sample and viewing in bright sunlight revealed that it was a deep red purple glass. A lighter version of this glass could explain the color known as “Oxblood” or “Fiery Amethyst”. The lightest color found was light lavender (3-D).
There are several reasons for variations in the color amethyst. The use of manganese had a number of drawbacks and was referred to as a “fugitive” colorant because it varies markedly in its coloring properties when subjected to continued contact with heat in melting, re-firing and in the annealing process. Glass had to be removed from the melting pot while a violet color in order to compensate for the “burning out” when annealed; so that when the glass cooled, it would be the proper color. Manganese was also referred to as “the Glassmakers Soap” since it has been used since about 1400 BC to decolorize or to “cleanup” the initial green color of glass to make it clear.
Over time and with sunlight (UV) exposure, manganese can chemically change giving the glass a light purple color (4-A). Manganese was eventually replaced with other colorants and may explain the difference in the color of amethyst carnival glass produced by the Dugan and the Diamond Glass Ware Company. Nickel oxide, which gives a purple to brownish color, may have replaced the manganese in the later-made amethyst of Diamond Glass.
Most collectors will probably agree that the Dugan amethyst is more desired than the Diamond amethyst. About 80% of Dugan/Diamond carnival glass patterns are found in amethyst. Photo 4-B is an example of Six Petals tri-corner bowl in Oxblood or Fiery Amethyst, 4-C is a typical amethyst Beaded Shell Mug, and 4-D is a Lavender Apple Blossom Twigs IC Bowl.
Amber - Four shades of amber and the color horehound.
Amber glass, also referred to as carbon-amber glass, started with the soda/lime glass formula, but without the use of nitrate as an oxidizer. The addition of a carbon source, such as sugar, was thought to be responsible for the amber color. Other carbon sources used were oats, corn, and powdered coal or coke. Most formulas had notes indicating that the addition of more carbon or a second carbon source would darken the shade of amber. Carbon-amber glass was first made in 1839; but it wasn’t until the late 1930’s, that chemists determined that the amber color was a result of sulfur (in the carbon) reacting with the iron impurities in the silica sand. Later, amber glass was made using only sulfur and iron compounds as the colorants and the intensity was controlled by the amount of alkali used in the glass. Other glass companies were known to make amber glass using uranium and selenium.
Four shades of amber glass ranging from dark to light amber are shown in Photo 5. The color “horehound” (5-E) appears to be a version of amber and is included here, although descriptions of horehound vary from brownish purple, grayed brown to root beer. This horehound-colored sample appears to be a mix of amber with a tinge of green. No specific formula for the “horehound” color was found in any reference. Most of the amber glass was made in the mid 1920’s in several shades of non-iridized amber glass. The main patterns were Adam’s Rib (1925) and the Victory Line (1928). About 4% of Dugan/Diamond carnival glass patterns are found in amber and/or horehound.
In Photo 6-A is a non-iridized dark amber Adams Rib Candlestick, 6-B is a dark amber Windflower bowl , and 6-C is a horehound Windflower Bowl.
The next article Transparent Colored Glass Samples – Part 2 continues with the transparent colors: yellow, green, blue and red.