The story of opalescent glass begins around 1870 in England although that is not to say that perhaps there were similar or simultaneous developments occurring in countries such as Germany, France or Italy.    The development of opalescent glass brought together the expertise of the chemist, the glass craftsman and the business entrepreneur. More will be said about the chemist and the entrepreneur later. Let's reflect back to the late 1800s to understand generally what was happening in the world & what the "Victorian Era" was all about. Queen Victoria had started her reign in 1837 as a 16-year-old and served her country until death in 1901. England's economy had changed from agricultural to an industrial base. Power had shifted from the aristocracy to the businessmen and engineers. Britain was the centre of a worldwide commercial system, which included active trade with the New England States. Steam and electricity had transformed every day life and the stagecoach had long since been replaced by the steam engine. The first telephone call was made in 1892 and four years later Marconi invented wireless communications. All the developments of the Victorian Era impacted lifestyles in North America about ten years after they occurred in England.
    
    In 1880 a significant development occurred in the American glassmaking history when Harry Northwood & his glass making talents, emigrated from England to the United States. Harry, from a long line of glass artisans, was a very talented young man & was astute in business. He quickly found that his "forte" was an ability to capture the expectations of the American glass consumer & then supply the exact items to satisfy the needs. The first of his many companies was founded in 1888 and the production of fine glass continued under his guidance until his death in 1923.
    In England the George Davidson & Co. was one of the more significant glass manufacturing companies, founded in 1867 when George recognized there was a shortage of oil lamp chimneys. In 1889 Davidson introduced one of their most successful and popular lines of opalescent glass, a colour which they called "pearline". It was only made in two colours, blue & lemon yellow, each with a white edge. Pearline glass was so successful that Davidson introduced new designs almost every year until 1903 & continued pearline production until the outbreak of the First World War. British glass is virtually always marked with a registration number which registers that pattern, identifies the manufacture & the date of registration.
This photo is a piece of Davidson's blue pearline, a novelty basket from 1893 in the "War of Roses" pattern with the registration number clearly embossed in the bottom, Rd.212684.
 Lemon pearline was often called Vaseline glass, which is the general term, used for any kind of yellow glass with an appearance similar to the old Vaseline ointment. Adding uranium to the glass mixture produced Davidson's lemon colour.  This biscuit barrel in lemon pearline glass is in the "1890 Suite" pattern or later named "Princess Diana" by Bill Heacock.
    There are three kinds of glass known as opalescent. One is blue-tinged, semi-opaque or clear glass with milky opalescence in the center. The colour is produced by the slow cooling of the molten glass in those parts that are thick causing some crystallization inside the glass. This contemporary opalescent glass was first produced in the 1920s and 30s by companies in France such as Lalique, Sabino and Jobling.
   The second kind of opalescent glass is hand-blown and is normally made from two layers of glass, the outer layer containing the heat-sensitive chemical.
    The third kind of opalescent glass has a milky white edge or a white raised pattern decorating a coloured pressed glass item. The effect is produced by re-heating parts of the molted glass just as it has started to cool. The heat sensitive chemicals in the glass turn the re-heated sections white. This article will focus on pressed opalescent glass.
    Now, about the chemist and chemistry of opalescent glass; In the laboratory, the chemist may have successfully found the perfect combination of ingredients to create a desire effect. Then it was up to the glass craftsman to accurately balance the batch mixture, the heat, the cooling and the physical limitations of molten glass. Added to this, in the early days, the chemistry of coloured glass was not totally understood. Occasionally unusual colours do turn up which are probably experimental pieces or the results of a batch going wrong.
    Coloured glass is produced by using various metal oxides. The colours vary according to the nature and quality of the oxides, the glass mixture into which it is added, and whether or not there is a reducing or oxidizing agent present. A reducing agent removes oxygen from a chemical while an oxidizing agent adds oxygen. Cobalt oxide gives a beautiful dark blue colour, manganese a purple or black colour and manganese used in conjunction with iron and arsenic will produce many shades of amber. Ferrous oxide produces olive green or pale blue. Ferric oxide will produce a yellow colour but requires the presence of an oxidizing agent. Copper gives a peacock blue colour, which can turn green if the proportion of copper oxide is increased. Gold is used for the production of red or ruby glass.
    By the late 1800s the glass industry in both England and America was growing & evolving very quickly. Pressed glass production had been automated & coloured glass technology had reached its peak. Combinations of new patterns & new colours were constantly being developed. The chemist worked behind the scenes & suddenly stumbles upon the formula to produce heat sensitive glass. The addition of arsenic, uranium or gold to the batch mixture gave the molten glass the ability to change from one colour to another when reheated at the glory hole. The glassmaker controlled the precise area of the colour change by re-heating only the desired part of the glass.
    At the same time, it was discovered that by adding bone ash to the glass, upon re-heating, the piece would turn a pearly white colour; thus the introduction of OPALESCENT GLASS. One record alone provides us with an insight into what was occurring in coloured, heat sensitive glass development. Thomas Webb & Sons Co. in England introduced 65 separate colours between 1875 and 1898, ten of the colours having reference to opalescence. Then came the first developments of Burmese glass. In America in 1883, Joseph Locke registered a patent for Amberina glass & three years later Peach Blow, a cream coloured glass with a heat sensitive overlay, had been developed. From its inception, opalescent glass has enjoyed a widely receptive audience. A young growing market was ready for any touch of brilliance and beauty to display throughout the home. The craze & frenzy to own & display a particular colour & pattern of glass perhaps was not unlike today’s “Beanie Baby" Phenomenon.
   The price was right, for example, 12 pieces of Beaded Fleur-de-Lis sold for the wholesale price of .80 cents a dozen or approximately .29 cents each retail.
   The major production of opalescent glass covered 40 years from 1880 to 1920. During that period the Fenton Art Glass Co. of Williamstown W.V. also started to manufacture opalescent glass.
Here is a good example of "Waterlily with Cattails", a 1905 piece in white but also made in blue, green & amethyst & later made in Carnival glass. Fenton Art Glass is still manufacturing opalescent glass today relying upon the heat sensitive technology developed 100 years ago.
   Let's relate the interwoven story of Carnival Glass, another heat sensitive product which is sprayed with
metallic salts & reheated to produce the characteristic shiny metal finish.
    The transition of pressed glass patterns is most interesting. Carnival Glass, introduced in 1905 obviously made use of previously popular patterns, for example Waterlily with Cattails. Paneled Holly & Beaded Cable appeared in both opalescent and carnival glass pieces.
  Here is a piece of peach opalescent carnival glass in the Question Mark pattern, probably made by Dugan Glass Co.
    Pressed glass designs started off with nationalistic symbols, roses, shamrock and thistle then along came geometric and abstract designs. At the turn of the century naturalistic designs evolved using fruits, vines, shells and fish. I think carnival glass exploded these natural design trends in America. Just looking at a collection of pattern glass reveals these design
trends, all brought about by competition to attract the consumer's attention. Clearly the manners, morals & design tastes of the Victorian Era still influenced life in America & continued until a period just before the First World War. The business entrepreneur had to be very nimble & almost gifted, to react, & position his business in the middle of popular glass trends. To complicate the situation he faced partner and key employee unrest that often led to people leaving, only to move down the road & start up a new glass-making venture. A review of old records indicates that the capital to start a plant was readily available. It was an ordinary event to divide up a proposed plant site into lots & sell them at a price of perhaps $200 each.
   A period of growing unrest and uncertainty among glassworks employees brought about organized labor under the banner of the "American Flint Glass Workers Union". The businessmen join forces to form a solid front against this movement and in 1891 formed the United States Glass Company. In 1899 a competing conglomerate was formed called the National Glass Company but only lasted for five years. Perhaps its demise was the apparent strategy to acquire financially destitute companies, close them up and move the moulds to larger more successful operations. By 1920 there were over two dozen glass factories large and small producing a vast amount of glass. Tableware in particular was available in every colour one could image and in affordable designs and styles to fit every taste and budget. In June 1999 I visited the Kokomo Opalescent Glass Co. in Kokomo, Indiana established in 1888. They also are relying on 100-year-old technology to manufacture rolled cathedral opalescent glass. A recent boom to their business is the growth in handcrafted stained glass windows, sun-catchers and novelties. When you have an opportunity to tour a glass manufacturing facility the tour seldom includes the batch mixing room, this is where all the secret formulas are stored. Not so in Kokomo and it was interesting to see batch-mixing carts with built-in computer scales to ensure that just the right amount of each ingredient was added to the batch. Kokomo's process did not include a re-heating step to create a pearly white colour in any one area.
   The object of the process was to create all the colour shades and variants of a base colour imaginable within a 3' x 8' sheet. It was a great experience to walk down an 80-foot long corridor with stacks of colour panels on both sides arranged into colour groupings.
   I spend a lot of time researching and collecting Greentown glass from the Indiana Tumbler and
Goblet Co. This company was in business from 1894 to 1903 a time period that corresponds with the development of heat sensitive glass. Jacob Rosenthal was the chemist that developed the formula for chocolate glass and golden agate. I think that heat sensitive glass technology has a place in his formula but surprisingly through all of my research and reading I have not encounter any reference to heat sensitive glass, opalescent glass and the name Rosenthal. Unfortunately golden agate in the "Holly" pattern was
only produced for a short period of time before fire destroyed the Indiana Tumbler and Goblet Company. Holly Amber is probably the most classic example of heat sensitive glass. The intricate and detailed design of the molds appears to be specifically created to attract the white opaque opalescent colour to the highlight areas. The batch mix must have been just right to create the very clear amber colours of the holly leaves, berries and vines.

   The story and events behind the development of "Heat Sensitive Glass" is one that can continue to lead the mind into many avenues of research and investigation. I hope this overview can do the same for any reader. I have greatly broaden my knowledge about glass and glass developments, I hope that the same can happen for you, HAPPY HUNTING.
REFERENCES

PUBLICATIONS: Hajdamach, Charles R., British Glass 1800-1914, 1991 Edwards, Bill, Standard Encyclopedia of Carnival Glass, 1991
Edwards, Bill, Standard Encyclopedia of Opalescent Glass, 1997
Hartung, Marion T., Opalescent Pattern Glass, 1971
Heacock, William, Opalescent Glass From A to Z, 1975
Heacock, William, Collecting Glass, 1986
Husfloen, Kyle, American Pressed Glass 1825-1915, 1992
Lattimore, Colin R. English 19th-Century Press-Moulded Glass, 1979

PERIODICALS: The Glass Collector, Winter 1983
Royalty & Empire, 1983
Glass Collector's Digest, August/September 1987
Glass Collector's Digest, August/September 1988
Glass Collector's Digest, December/January 1991

links to RELATED INTERNET WEB SITES:
www.kog.com
www.encyclopedia.netnz.com/opalescentglass
www.glass.co.nz/Davidson.htm
Another Interesting Article on Opalescent Glass