Colorless is preferred over the term "clear" or "white" glass since the former term refers more accurately to the transparency of the glass not its color, i. Colorless glass was a goal of glass manufacturers for centuries and was difficult to produce because it required the use of virtually impurity-free materials. Venetian glass makers produced their crystallo as early as the 15th century and glass makers in 18th century England made what was known as "flint" glass from virtually pure quartz rock calcined flint which was simply called "flint" Hunter Improved chemistry and glass making methods of the late 19th and early 20th century allowed for process efficiencies that made colorless glass easier and much cheaper to produce with the use of various additives in the glass mixture.
The term flint glass was and still is used somewhat erroneously by glassmakers to describe colorless glass that is made with low iron sand. It is, however, not true flint glass.
Colorless glass was also called "crown" by early glassmakers Hunter Colorless glass is not always, or even usually, absolutely colorless. It will usually have very faint tints of pink or "amethystine" faintly visible in the base of the bottle to the left , amber or "straw", grayish green, grayish blue, or grey. These faint colors are viewed easiest when looking through the thickest portion of the bottle, i.
Colorless glass is usually attained by using the purest sand source possible and by adding "decolorizing agents" to the glass batch to offset the residual iron impurities Dillon Common decolorizing agents were manganese dioxide, selenium dioxide usually in conjunction with cobalt oxide , and arsenious arsenic oxide which is also used as a stabilizer of selenium in decolorizing glass - or some combination of these compounds Trowbridge ; Scholes ; Tooley ; Lockhart a.
Colorless glass actually does have more utility in dating and typing than most other colors, though still of limited application. Some of the better dating reliability is for bottles with manganese dioxide decolorized glass. Upon exposure to sunlight, this glass will turn a light pink or lavender to moderately dark amethyst or purple depending on the amount of manganese and amount of ultraviolet UV light.
This is called "sun-purpled" or "sun colored amethyst " SCA glass. This bottle began its life as colorless glass and has "turned" a much darker than average color of amethyst most likely due to the application of artificial UV light or other artificial source like sterilization equipment , i. The light lavender tint produced by manganese offsets the green tint of the iron impurities in sand creating a largely colorless glass. For an interesting article on the artificial irradiation of historic bottles, see Dr.
Cecil Munsey's article available at this link: Manganese became known as "glassmakers soap" due to the ability to "cleanse" or neutralize the effects of other impurities in the sand, particularly iron Hunter Manganese dioxide induced colorless glass was, however, by far most commonly used from the s to about the end of World War 1. At that time manganese dioxide use was greatly reduced for a variety of reasons, although in part because it did not work as well as other chemical decolorizers see next paragraph in the open continuous glass tanks used by the increasingly dominant bottle making machines - both semi-automatic and automatic.
It is often noted in the literature that the reason for the switch from manganese dioxide to other decolorants was due to the cut-off of imports usually from the Caucasian Mountain region of Russia to the U.
However, it is more complicated than that with other reasons being wartime allocation of the now scarcer manganese to the more important need for producing steel and the dramatic increase in the application of chemical knowledge to glass manufacturing which lead to the eventual realization that glass batch mixes with selenium as the primary decolorizer simply worked better Peter Schulz, unpublished manuscript Colorless glass which was de-colorized with selenium or arsenic or typically a combination of the two in conjunction with cobalt oxide results in a very faint "straw" or amber tint to the thickest portions of the glass Scholes ; Tooley ; Lockhart b.
The picture to the left shows this color evident in the thick portion of a milk bottle underneath the line pointing out the valve mark that dates from between and based on a makers mark for the Pacific Coast Glass Company Toulouse Click Cloverdale Dairy Co. This colorless "color" can be very diagnostic of a machine-made bottle made from about to to typically no later than the s Girade ; Lockhart b; empirical observations. The straw tinted colorless glass in bottles does show up frequently in mouth-blown bottles but typically later ones , although can be found occasionally in bottles from the midth century.
Click French mustard bottle to view an s era bottle with a faint straw cast - evident at the heel - to the otherwise colorless glass. Selenium was the best decolorizer for glass made in open glass tanks versus the earlier closed pots which was used with most all automatic bottle machines.
Both of the above colorless glass tints can be useful diagnostic tools for an archaeologist who may be dealing with fragmental bottles. One can be quite confident that if the fragment is colorless with a slight straw tint, it very likely is from a machine-made bottle, unlikely to date from much prior to World War 1 i.
Conversely, a colorless fragment with a slight amethyst tint is quite likely to date to or prior to World War 1 and is more likely than not to be from a mouth-blown bottle. Bottles with a grayish tint seem to date between and , although numerous examples outside that range have been noted by the author Giarde ; empirical observations. Generally speaking, bottles of colorless glass were relatively uncommon prior to the s but became quite common after the wide spread use of automatic bottle machines in the mid to late s Kendrick ; Toulouse a; Fike ; U.
Nothing is absolute in these date range estimates, but they are believed to have reasonably high reliability. Return to the top of this page. Aqua Aquamarine This color - like most of the colors that follow - had many subtle variations and shades. The "gothic" or "cathedral" style pickle bottle ca. Shades of aqua are a very common color for these utilitarian food bottles which were a common stylistic design particularly between the s and s though some examples date before or after that period.
The term aqua is a preferred by this website shorthand version of the term aquamarine. Use of modifying terminology is frequently employed to make more precise the color shape, intensity, or hue.
For example, the fruit jar pictured below would be considered deep blue aqua. Aqua glass is a "natural" result of the iron impurities found in most sands. It is very rare maybe unknown that sand does not contain some traces of iron.
Sand deposits with very low iron content were and probably still are highly valued commodities. Although good quality sand was plentiful in the Eastern United States, some was still being imported from Belgium for Western American glass factories as late as the s. Aqua glass is the result of sand which is relatively low in the amount of iron which was not off-set by de-colorizing agents as noted in the colorless glass discussion above.
High levels of iron produce darker greens, black glass, and even amber. Natural aqua glass was often called "green glass," "bottle glass," or "bottle glass green" by glass makers Kendrick ; White Different shades of aqua and the related blue-green colors which are often observed in the same bottles blown in the same mold may be explained - at least in part - by the following information quoted from Julian Toulouse's excellent book A Collector's Manual - Fruit Jars a.
This excerpt is making reference to some of the effects - desired or not - that occur when mixing and melting glass. It also points out one of the many complexities inherent in producing desired glass colors: Little was known about the influence of the flame. A "reducing" flame, or one with less oxygen supplied for burning, might produce a bluish-green because the iron in the sand might then be reduced to one of the bluer iron oxides - an excess of air might make the oxidized green iron oxides predominate.
Early glassmakers knew little about this. Thus a fire banked for the night and with the air intake flues closed down, could produce quite blue glass for the morning's start, and change slowly during the day when the air vents were opened wide for a hotter flame. Aqua is a very common color in all types of American made bottles that date prior to the s back at least to the early 19th century.
Thus, no specific type or class of bottles is more or less apt to have been made with aqua glass. The one significant exception to this dating is soda bottles, e.
Shades of aqua color survived in soda bottles long after the s and is still found today where glass is still utilized for soda containers. Another notable exception is that many fruit jars were also made well into the the s in aqua though colorless glass probably became the majority color by about Ball fruit jars were made in a distinct "Ball blue" from at least until at least the late s and even later for some specialty items Creswick The picture to the left shows two sizes of Ball's very popular Perfect Mason fruit jar.
This color is light but a more intense blue than blue aqua but does not quite fit the other color groups described below; thus its coverage here. In , the Ball Company was making This market domination during the first half of the 20th century explains the commonness of these fruit jars today and in historical sites from the early s through the Great Depression.
Opaque White or Milk Glass Opaque white glass - commonly called milk glass but sometimes called opal or white glass - was typically produced by the addition of tin or zinc oxide, fluorides fluorspar , and phosphates Illinois Glass Co.
In a sense, milk glass the preferred term on this website is like colorless glass in that it is defined by the absence of color, except in this case the bottle is truly not "clear". An interesting feature of most milk glass is that very thin glass i. It was also reputedly hard on the longevity of glass melting tanks and pots so was avoided by some glass factories. Milk glass was used in the production of a wide array of different type bottles, though there is some typing and dating utility to the color.
A typical cosmetic bottle example is the Owl Drug Company lotion and likely other cosmetic products bottle to the left which dates from the around Jensen ; Fike Many of the milk glass druggist type bottles were most likely for that particular druggist's cosmetic products as milk glass was well linked with cosmetics in customers minds Fike ; Cannon Milk glass was occasionally, though uncommonly, used for ink, bitters, non-cosmetic medicinals, liquor, and sometimes even fruit and food jars primarily during the late 19th and early 20th century.
It was rarely used for bottles prior to about , i. An interesting aspect of some milk glass is that it was made with manganese dioxide in the glass batch. This milk glass will react to sunlight - as described in the colorless glass section above - producing a milky lavender color. This has primarily been observed in cold cream and other cosmetic jars from the first couple decades of the 20th century, though it is seen in occasional bottles.
It sometimes appears that the lavender color was produced purposefully with a UV light or radiation so it is not always possible to differentiate between sun colored and intentionally colored lavender milk glass. It rivals the multitude of amber glass variations which, as noted below, can grade into various greens. The different greens were formed by a myriad of different coloring agents, impurities, and glass making processes. Iron, chromium, and copper all produce different green glass.
Chromium oxide will produce yellowish green under oxidizing conditions and emerald green under reducing conditions in the glass furnace Dillon Combinations such as cobalt blue mixed with chromium green will, not surprisingly, produce blue-green glass Kendrick ; Munsey Just as there were many ways to produce different green glasses, there are endless naming variations for the green colors, e.
As in describing all bottle colors, modifiers are useful in clarifying the specific greens. For example, the flask to the left would be considered a medium blue-green with a slight slant towards the green end.
This early American New England flask has an eagle motif on one side, a Masonic emblem on the reverse, and was produced ca. The mineral water to the right could be described as medium to dark emerald green. This color is distinctive to mineral water bottles blown at the Congressville NY Glassworks and it and some subtle variations are known to collectors as "Congressville" green.
This distinctive style of bottle is often referred to as a "Saratoga" style Tucker Compare this color to the medium blue-green with a slant towards green flask in the upper left corner of this box. This bottle was produced during the era when applied finishes dominated, though it has an usual - for the time - tooled finish. The flask below has a color that could be described as clear yellow-green or possibly light to medium emerald green.
The bottle to the lower right is an example of what some call citron after the color of the fruit of that name. This color is generally a "brilliant greenish yellow" like the pictured bottle though citron is sometimes described as a "brilliant