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Lead Oxide as an Ingredient in Glass

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An original palm kohl-tube, c.1400-1225 BC. From Goldstein, S. (1979) 'Pre-Roman and Early Roman Glass in The Corning Museum of Glass' Corning: New York, cat. no. 24, inv. no. 71.1.4, showing lead antimonate yellow decoration Lead in Ancient Glasses:
Lead has been an ingredient in glass as early as the Egyptian Bronze Age, when it was used in conjunction with antimony to produce a lead antimonate opaque yellow. Core formed vessels of the Eighteenth Dynasty (pharaohs including Ahkenaten and his son, Tutankhamun - the mid to late 14th century BC) were often decorated with lead antimonate yellow, along with calcium antimonate white and cobalt and copper blues.

A ribbed bowl from Vindonissa, Switzerland, c.1st century AD, showing lead antimonate yellow decoration Lead antimonate yellow remained in use throughout the 1st millenium BC: it is often seen on core formed vessels and glass inlays and beads of the 5th century BC and later, and on glass vessels from the Hellenistic and Republican and early Imperial Roman periods. More rarely, it is seen on later Roman glass, such as 'Snake-thread' glass vessels from the 4th century AD.

Although lead antimonate and lead stannate opaque yellows were used by Venetian glassmakers of the Italian Renaissance, lead has been superceded by other materials, such as cadmium sulphide.

Lead is often present in opaque whites from the Hellenistic and Roman periods, notably in analyses of 1st century Roman cameo glass, such as the Portland Vase and the Auldjo Jug - both in the British Museum. The white glass of the Portland Vase body has a lead content of 12%, and that of the Auldjo Jug is 22% (Bimson and Freestone 1983, Freestone 1990).
High-lead glass goblet (13th-14th century) from Old Sarum Castle (photo: Salisbury and South Wiltshire Museum) Medieval Lead Glasses:
The earliest surviving treatises on medieval glass are from the 10th and 12th centuries.

Eraclius, 'De coloribus et artibus Romanorum', from the 10th century, has a chapter added in (probably) the 12th century: ‘How glass is made of lead, and how it is coloured’. He writes of a high-lead glass of two parts lead to one part sand.

Theophilus, 'De diversis artibus', from the 12th century, mentions lead glass finger rings, and a missing chapter apparently refers to using lead in (glass?) pigments.

Examples of early lead glass, dating from the 9th to the 13th centuries, have been found in England, the Low Countries, Germany, Sweden and Southern Russia, and are thought to have been produced in Northern Europe and Southern Russia. Many of these are high-lead glass objects (finger rings, bangles, beads, gaming counters, mosaic tesserae and jewellery inlays) and vessels containing from 60% to 80% lead. High-lead objects and vessels from Kiev contain up to 64% lead, but other glass fragments from Kiev have a lower content of between 20% and 30% (Tyson 2000, pp.11-12).

Tyson lists a group of nine or more high-lead glass goblets, with a typical transparent yellow colour due to the lead content, from sites in England dating from the 13th and early 14th centuries, including an incomplete example from Old Sarum, with parallels from Germany containing up to 84% lead (Tyson 2000, pp.17, 59-61, fig.6, Types A5 and A6). She also lists other high-lead glass tableware, including beakers (ibid. pp.82-85, fig.12, Types B10 and B11), flasks or jugs (ibid. pp.117-118, fig.20, Type D4) and some miscellaneous fragments (ibid. pp.185-186, fig.37, Type H16).
Detail from a late 18th century opaque twist stem English and Irish Lead Crystal (Flint Glass):
Lead crystal was commercially developed by George Ravenscroft in the late 17th century, and in 1674 he was granted a Royal patent to produce and sell glass vessels made from it. The term 'flint glass' was in use because the original source for the silica was calcined flints, and does not necessarily mean that lead was a component of the glass.

Recipes for high lead glasses had been written down by this time. In 1651, Glauber published his 'New Philosophical Furnaces', containing recipes for high lead glass and a glass made by combining high lead glass with 'Venice glass', and both Newton and Boyle noted recipes down in their laboratory notebooks. In 1662, Merret published a copiously annotated translation of Neri's 'L'Arte Vetraria (Art of Glass)' of 1612, which contained recipes for high lead glass for imitation jewels.

Ravenscroft may have made use of this information, helped by an Altarist glassmaker, Johan van Barmont Baptista Da Costa, who had peviously worked in Nimegen with, amongst others, Jor Odacio, also from Altare. Odacio established a glasshouse in Dublin and applied for and was granted a similar patent to Ravenscroft's in 1675. Both of these glassmakers may have brought the knowledge of glassmaking using lead to their respective new homes (Brain 2008, p.112; Dungworth and Brain 2009, p.112 (also!)).

The glass Ravenscroft used for his first products was chemically unstable and within a short time started to crizzle (devitrify). He corrected this by increasing the lead oxide content, thereby creating a more chemically stable glass.

Over the years of the late 17th century, increasing amounts of lead were added to glass, both during the Ravenscroft period in order to counter the crizzling problem, and after he had retired from glassmaking in 1679 (although his brother, Francis Ravenscroft continued at the Savoy glasshouse in London until the patent finally expired in 1681). Dungworth and Brain have identified four groups of late 17th century lead crystal compositions, based on lead content vs. potassium content, both during (Group 1 (16.4% lead) and Group 2 (27.2% lead)) and after (Group 3 (34.5% lead) and Group 4 (39.9% lead)) the Ravenscroft period (Dungworth and Brain 2009, p.125 et seq.).

There is evidence that during the 18th century, the lead content in lead crystal settled to between 34% and 40%. In a small set of twenty samples of 18th century glass, lead content averaged 37.38%, with a range of 34.11% to 39.95% (Colin Brain: pers. comm.). The first tax on glass, introduced in 1695, was calculated on the weight of the glass produced at each glasshouse (as was the second tax, imposed between 1745 and 1845), and this appears to be the reason for the reduction in the amount of lead at the end of the 17th century.

Quick calculations on recipe nos. 57, 61, 68, 71, 78 and 81 (chosen at random) from Gillinder (Gillinder 1851) give an average lead oxide content of 34%, an indication of the normal lead crystal compositions during the 19th century.

During the 20th century, and into the 21st century, two commonly used amounts of lead in lead crystal have been 24% and 30%. Lead contents of 30% and more are known as 'English Full Lead Crystal'.
Properties of Lead Crystal Glass and Use of Lead in Glassworking:
Lead, added as litharge (PbO) or as red lead or minium (Pb3O4) imparts several useful properties to the resulting glass.
  • The glassblower and his assistants benefit from a glass which stiffens slowly as it cools (it has a long working range)
  • It anneals at lower temperatures than soda-lime glass (the general composition of ancient glass), which saves on fuel
  • When cooled, the glass is softer, making it easier to cut and engrave
  • It increases the refractive index of the glass, making a heavily-cut colourless vessel appear more 'brilliant'
  • It increases the weight of the glass
  • It allows a long-lasting note or 'ring' when the rim of a bowl is tapped
In addition, when a finely-ground glass containing lead is used as an enamel, the lead lowers the 'firing', or softening temperature of the enamel, and depending on the amount added, allows the enamel to fuse to the parent vessel without the vessel distorting during re-heating.

Lead oxide has been a traditional material for marking out and polishing cut and engraved glass.

In the 18th century, it was also used as one of the ingredients of the glue used in gilding drinking glasses (Charleston 1972, p.22).

A modern use for lead in glass is in radiation shielding.
Disadvantages:
There are several disadvantages to using lead in glass:
  • The ingredients of lead crystal, mainly high purity silica sand, potassium (carbonate and nitrate) and lead oxide itself, have become far more expensive over the last few years (2007 - 2011), and the prices will, no doubt, continue to rise
  • Health and safety regulations have necessarily become stricter, and the glassworker has to take precautions when working with lead oxides. Lead can be absorbed into the body as a consequence of eating, breathing and touching chemicals containing lead. Once absorbed, it is difficult, if not impossible, to remove.
  • Adequate ventilation has to be used when melting and working with hot lead crystal or when grinding or engraving cold lead crystal
  • If lead metal reduces out of the molten glass, it will migrate to the base of the pot and very quickly attack the pot fabric, causing holes in the pot. To prevent this from happening, an oxidising agent is added - usually in the form of potassium nitrate (salt petre).
  • There is evidence that lead can be absorbed from the wall of a vessel into liquids such as wine, spirits and citrus juices, and this is an important consideration when using lead crystal drinking vessels and decanters. In particular, these liquids should not be stored for long time periods in lead crystal decanters: far better to only use decanters to bring the liquids to the table and to pour out any left-over liquids.
Much has been written on the dangers of lead in the environment, and on describing the symptoms and effects of cumulative absorbtion of lead. Detailed information can be found at these links:

UK health and safety website        UK medical website

USA government website        Canadian government website

We have also made available a couple of articles which specifically mention lead in glass and the effects of drinking from lead crystal glasses or storing wine, spirits, fruit juices and other liquids in lead crystal decanters:

Lead Crystalware and your Health (Health Canada 2003) – recommends soaking new lead crystal in vinegar for 24 hours before using it as it leaches lead from the surface of the glass, and only washing it by hand, using a mild detergent.

Lead, Glass, and the Environment (Hynes and Jonson 1997) – also mentions the leaching property of vinegar. Of the leaching by wine, they write (p.144):

"Typical 24% PbO crystal currently manufactured can be expected to leach ca.10pg Pb during three 1hr contacts of 150ml of wine per contact (a total of 450ml). Most of the leaching occurs during the first two contacts so that successive contacts will not significantly increase the lead intake. These conditions represent an extreme case in view of the quantity ingested and the contact time. However, such an intake does not present any undue risk to the average adult, particulary in view of the fact that lead absorption during meals is less than 10%." NB 1pg (1 picogram) is equal to one trillionth of a gram (10-12gm).

One has to remember, though, that there are many other hazardous ingredients used in glassmaking, and they all require sensible handling. Both hot and cold glassworking have their dangers, whether or not lead is used.
'Dr Syntax in the Glasshouse' (1820) by Thomas Rolandson, from 'The Second Tour of Dr Syntax in Search of Consolation'

'Dr Syntax in the Glasshouse' (1820) by Thomas Rolandson, from 'The Second Tour of Dr Syntax in Search of Consolation'
References:

Bimson, M. and Freestone, I. C. (1983) 'An Analytical Study of the Relationship Between the Portland Vase and Other Roman Cameo Glasses' Journal of Glass Studies Vol.25, pp.45-54. The Corning Museum of Glass: Corning, New York

Brain, C. (2008) ‘Vitrum Saturni: Lead Glass in Britain’ In von Kerssenbrock-Krosigk, D. ‘Glass of the Alchemists: Lead Crystal/Gold Ruby, 1650-1750’ pp.106-121. The Corning Museum of Glass: Corning, New York

Charleston, R. J. (1972) 'Enamelling and Gilding on Glass' The Glass Circle Journal Vol.1, pp.18-32. Oriel Press: Newcastle upon Tyne

Dungworth, D. + Brain, C. (2009) ‘Late 17th –Century Crystal Glass: An Analytical Investigation’ Journal of Glass Studies Vol.51, pp.111-137. The Corning Museum of Glass: Corning, New York

Freestone, I. C. (1990) 'Laboratory Studies of the Portland Vase' Journal of Glass Studies Vol.32, pp.103-107. The Corning Museum of Glass: Corning, New York

Gillinder, W. (1851) 'The Art of glass Making' Gillinder: Birmingham

Health Canada: www.hc-sc.gc.ca/index-eng.php

Hynes, M. J. and Jonson, B. (1997) 'Lead, glass and the environment' Chemical Society Reviews, pp.133-146

Tyson, R. (2000) 'Medieval glass vessels found in England c. AD 1200-1500' Council for British Archaeology: CBA Research Report 121

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