Lime and Lead

Lime & Lead

Reproduced and updated from Glass Circle News 100th Edn. 2004 Centenary Supplement.
by David C. Watts

Glass is a living material in as much that it undergoes continuous change in the furnace due to the temperature, the atmosphere, contact with the pot and the time it is there. To these must be added the composition of the batch used and the physical consistency and purity of the ingredients.

Our scant knowledge of how the ancient glassmakers handled these problems comes from the famous inscribed Assurbanipal clay tablets dating to around 1400 BC or even earlier after which, with the odd notable exception, there is a gap of some 2000 years before published accounts of the glassmaking process begin to emerge in the 15th century. Yet during this time a remarkable diversity of glass objects were produced with equally diverse compositions. The body of the Portland vase, for example, contains lead but the disc in the base, although looking much the same, contains no lead. Does this mean that there were two glassmakers of the period capable of creating this difficult overlay glass? The lead seems unlikely to have been a contaminant and could have been added deliberately to soften the glass for the carver It implies a greater understanding of the glass technology at that time than we might have thought possible.

The fortuitous simplicity of the fact that certain plant ashes contain ingredients that when combined with sand and heated will give glass may lead us to think that the ancient glassmakers were incapable of, or never bothered to, find out what they were. This, in spite of the fact that they discovered most of the colouring agents, sophisticated clarifying agents, such as antimonates, and even the use of lead in a relatively colourless glass that we use today.

A simple glass consists of water-soluble sodium and/or potassium silicate rendered insoluble by the presence of calcium added in the form of lime or chalk; in lead glass the lead replaces the calcium. The first translation of the Assurbanipal tablets by Campbell Thompson posed the great problem, taken up in a questionable analysis by W.E.S. Turner, that the recipes used by the ancients failed to recognise the vital role of the calcium. The argument hinged on Campbell Thompson's interpretation of the transliterated word "namrutu" (literally "white stuff"). He concluded that it meant chalk or lime by a process of word elimination. Great hopes of resolving this problem emerged with a new translation by A. Leo Oppenheim but while the understanding of the organisation of the tablet fragments and other peripheral matters (from a purely glass viewpoint) were improved the crucial word "namrutu" remained untranslated. Confirmation, and in some places, extension, of the Cambridge Don's achievement were welcome but the improvements in terms of glass chemistry were disappointing.

A seminal paper by two American chemists, Sayre and Smith, added a new complication. The Egyptians were found to have been making a glass that did not involve plant ash; minor components characteristic of plant ash, particularly magnesium, were missing from their analyses! The new source of soda was tracked down to the calcium-free salt lakes of the Wadi Natrun. So the question now became what is the source of the calcium in these glasses? Currently, our archaeologists seem to have resolved this conflict to their satisfaction by suggesting that plant ash glass is made with sand that lacks calcium (traditionally from an area on the shore of the river Belus that has never been identified) while the Wadi Natrun glass was made with sand adequately contaminated with limestone of which there is indeed no shortage in the Middle East. Is it credible that the ancient glassmakers, capable of making glass by two such different methods, remained unaware that the mystery component even existed? Their understanding of chemistry may have been primitive but they were not stupid. Further, we now know that the glass-maker could assess the quality of his plant ash in terms of its "namrutu" content by its sharp taste, as mentioned by Pliny the Elder and visibly demonstrated in Robert Brill's famous film, The Glassmaker of Herat (The glass workshop still exists [in 2004] in Afghanistan).

Adding too much calcium to the batch both raises the melting temperature and renders the glass stringy and unworkable. So it is not a component to be added incautiously if the amount already present is adequate. The Assurbanipal tablets suggested that the ancients did add small amounts of calcium in the form of shells and coral to their batches which might have been to "fine-tune" the mixture — a thought not put to us by W.E.S. Turner. The problem emerges in reverse in mid-15th century Venice. Glassmakers there began to purify the soda from plant ash in an attempt to achieve the much desired imitation rock crystal. Purification involved mixing the ash with water to extract the salts; the liquid was then decanted or filtered off and the soda crystallised by evaporation in large pans. Because lime is almost totally insoluble in water it is left behind with the residue. A recent (2001) publication by professor C. Moretti and T Tonninato records the process described in "Tuscan Trattatello", probably dating to the 1st half of the 15th century, one of five early glass recipe books (spanning 1536-1644) examined by them. The discovery of cristallo is traditionally attributed to Angel Barovier but the invention is now thought to originate much earlier. Moretti's recipe book states that "crystal in all perfection" is simply prepared from a mixture of the purified salt and (crushed) pebbles without reference to any problem caused by the absence of the lime stabiliser. So why is the necessary addition of lime not mentioned? The authors join other commentators in endeavouring to resolve this difficulty. One, unlikely, possibility is that lime stones were accidentally collected along with the quartz pebbles. More probable explanations are that the batch, after being fritted, was then mixed with ordinary frit, prepared from crude (lime containing) ash, to make the final crystal, or that some of the lime sludge from the purification was added. Much early cristallo is known to have suffered from heavy crizzling, most probably resulting from the minimal amount of crude frit or lime sludge used to achieve an adequately stable glass at the time of manufacture. It is not difficult to clean up the lime, the major component in leached ash, by further washing. It emerges as a clearly visible white powder representing some 98% of the insoluble matter that any glass researcher can prove for himself. But if this fairly obvious solution was adopted the recipe books do not say so and the question of whether the old glassmakers really knew about the necessity for lime in the batch remains a question of personal opinion.

Moretti's recipe books also reveal that early Venetian lead glass seems to have been made in much the same way as "crystal in all perfection." Calcined lead (litharge or red lead) was fused with crushed pebbles to make a yellow lead silicate that was then mixed with ordinary frit to make the final glass. Hence this lead glass contained some lime and other materials, such as magnesium and alumina, derived from the ordinary frit. Reference is made to its occasional artistic (blown) use but its handling properties did not favour Venetian taste of the time and it was employed almost exclusively for paste jewellery in a variety of colours. The other use of lead at this time was in combination with arsenic or antimony salts to make opaque glass, a process going back to antiquity. Not until the late 17th century did lead come into more general used for Venetian blown glass when Briati introduced it in coloured glasses, particularly chandeliers, in styles exploited by Salviati in the 19th century.

Meanwhile, by the end of the 16th century the north European glassmakers had developed an unspecified "white glass" in contrast to the green waldglas or verre de fougere. This was exploited by Caspar Lehman for wheel engraving (tiefschnitt) alongside predominantly Dutch diamond point decoration on green glass. Ravenscroft's invention of lead crystal, in 1674, had little impact here, unlike Michael Muller's 1683 discovery of chalk glass (both chalk and lime are insoluble salts of calcium) in what is now the south Czech Republic. This invention kick-started heavy engraved relief decoration (hochschnitt) in the glass centres of Germany and Saxony, exploited in a limited way on a few English lead crystal glasses of the baluster period in the first quarter of the 18th century. Modest attempts seem to have been made on the continent to imitate English lead crystal, possibly up to circa 1718 by the Bonhomme glassmakers but the evidence is far from conclusive. From 1714, the ruthless suppression of luxury table glass manufacture in the Netherlands by the Habsburg princess, Maria Theresa, following the Spanish War of Succession, allowed chalk glass imported from Bohemia to dominate the first half of the 18th century on the Continent. As a result of her imposition, effected by means of heavy import taxation, facon de Venise glass and English lead crystal made only modest inroads during the first half of the century, although the latter became more dominant in the last quarter of the 1700s as factories in France and North Holland discovered the secret of its manufacture.

Exactly how English lead crystal was invented is likely to remain a subject for debate. Ravenscroft lived at a time when every gentleman took a wide interest in scientific matters and the influential Royal Society was little more than a decade old. Further, he had spent time in Venice, had Merrett's English translation of Neri available (if not the others already mentioned, since he presumably spoke Italian?) and, as a successful "Turkey Merchant" had money to spare on a speculative enterprise. An assistant glassmaker was, however, essential and much has been made of the role of Da Costa in the discovery. He had worked with Jean G. Renier and John 0. Formica in Nijmegan and was familiar with lead glass for jewellery as mentioned above. Why this trio split up we do not know; but one thing is certain, had they collectively resolved a reliable formula for lead crystal we would not have seen the expensive fiasco of the failure of Ravenscroft's first patent in 1674. Further, because the Savoy lay outside the City of London west gate it would not have been bound by the law prohibiting the use of coal. Whether the now typical English coal-fired furnace with closed pots, that Ravenscroft knew at Vauxhall, would have been preferred to the wood-fired continental furnace we do not know. But there is no question that he was clearly aware of the benefit of including saltpetre (also called nitre, potassium nitrate) in the batch. This was common knowledge among English glassmakers from Mansell's day. The original purpose of its incorporation was to consume the smoke from the coal-fired furnaces that darkened the glass. Saltpetre was stored in warehouses on the South bank of the river Thames which was very convenient for the glassmakers working there. (Its main use was, of course, for making gunpowder for the guns of the ships in Woolwich docks.) For Ravenscroft it was crucial to prevent metallic lead from destroying the pot. Da Costa may have been familiar with the early Venetian recipes and could have contributed to the early inclusion of borax as a flux, mentioned in the Giovanni Darduin recipe book of 1644 (see Moretti and Tonninato, above) but the inclusion of saltpetre is nowhere mentioned. On the other hand the Venetian recipes for jewellery record the use of 18%-20% (percentage by weight) of lead oxide in the batch and the destructive effect of molten metallic lead on the pot was well known.

Ravenscroft's original intention may well have been to make paste jewellery but on discovering to his surprise and delight that saltpetre protected the pot in some way he was tempted to experiment along the lines of Neri's much-proclaimed lead glass. The earliest Ravenscroft glass I was able to measure (from a fragment taken from the already broken stem of a lightly crizzled sealed roemer in the Victoria and Albert Museum, kindly provided for me by Robert Charleston) had about 10% by weight of lead, well below the levels used by the Venetian glassmakers. This suggests that the initial addition had been made with considerable caution and not foreknowledge of the outcome. It begs the question as to whether lead was used at all in his first endeavour. None of this could have been known a priori by Renier or Formica but there is no doubt that the secret, once discovered, would have been rapidly spread by Da Costa to his friends (assuming that their parting had, indeed, been fraternal!)

Had Ravenscroft not sent a large parcel of his lead glasses to Ireland the case for its independent early discovery and manufacture there by Formica would have been greatly strengthened. As it is, the merese found in Irish examples, not common in his English glasses, could simply have been added as an identifier by Ravenscroft to satisfy the Glass Sellers with respect to his contract with them. Nor should it be ignored that neither Renier, in Sweden, nor Formica in Ireland made claims for a patent until a year after Ravenscroft had overcome the initial problem of crizzling. In spite claims to the contrary, for me the balance of probability remains overwhelmingly with Ravenscroft, who was already involved in glassmaking at Vauxhall, as the originator of English colourless lead crystal for blown tableware. It is a clear case of the chance events of past English glassmaking favouring his well-prepared mind, the benefits of which Da Costa was able to exploit on his behalf.