Glass Manufacturing Technologies in Post-Medieval Europe
Glass has been manufactured in Europe for several millennia. In the post-medieval period (16th to 20th centuries) glass has been produced and applied in three main sectors: tableware (drinking cups, etc), flat glass (windows and mirrors) and containers (mainly bottles). Each sector has, at various times, had different requirements and found different technological solutions, although each has tended to borrow solutions developed in other sectors.
In post-medieval Europe almost all glass was made using a silica base with the use of a suitable flux (usually an alkali) to reduce the melting temperature. The use of different materials as the source of these fluxes is discussed in many historical sources for the period but can also been uncovered through the chemical analysis of surviving glass. Until the end of the 18th century, almost all fluxes used in glass making were obtained from plants. Many different types of plants could be (and were) harvested and burnt to produce an alkali-rich ash. Most of these may be divided into two broad groups: soda-rich plant ash derived from coastal and/or arid environments around the Mediterranean sea and potash-rich plant ash derived from temperature forests of central and northern Europe. Some contemporary glassmakers were aware that these plant ashes contained only a proportion of alkali and developed processes to purify plant ashes and so increase their alkali content. There was no real appreciation that sodium and potassium were separate alkalis until the late 18th century. The identification and isolation of these elements and the developments of techniques to allow their industrial production revolutionised the glass industry. Traditional plant ashes usually contained sufficient calcium to ensure that the finished glass was chemically durable. The role of calcium as a glass stabiliser and its deliberate addition for this purpose only developed from the late 18th century.
A significant driver for the post-medieval glass industry has been the production of colourless glass for drinking vessels; the goal seems to have been for a glass which would resemble rock crystal. Avoiding the production of any colour or tint in glass depends mainly on the use of raw materials which contain little or no iron. Most plant ashes contain sufficient iron to give resulting glass a colour. The use of lixiviation and other techniques to purify the flux largely removed this iron. The careful selection of suitable sources of silica ensured that glassmakers could produce truly colourless glass. Much of the early work to achieve this end took place in Venice but the late 17th century saw important developments in both Bohemia and England.
While tablewares had high status and have attracted most attention from later scholars, the production of utilitarian containers were a much larger sector of the industry. The post-medieval period saw the emergence of the ‘English' bottle which came to dominate container glass production. While bottles had existed before, the ‘English' bottle was thick-walled and narrow-necked allowing bottles to be effectively sealed and so allow their use for storage and transport. The nature of the glass employed in the production of bottles in England was for many years constrained by a system of taxation which allowed the use of only the cheapest raw materials. The progressive increase in the use of low-quality ingredients can be seen in the chemical composition of surviving glass bottles.
