Glass in Scottish Archaeology

It is only relatively recently that glass has appeared with any regularity in excavation reports from Scottish sites. To be fair, some sites yield many shards while others yield nothing. It may be that this inconsistency, especially when compared with pottery, has deterred more intense study of the subject.

Grisaille from Arbroath Abbey: Grisaille was a decorating technique where red oxide was used to paint fine design on plain window glass and then fired in. -This example is probably late 13th century.

In most cases the occurrence of glass depends on two things, date of site and social status. The earliest glass, really a proto glass, found in Scotland is Bronze Age faience where vitrification is incomplete either by design or due to low furnace temperatures.

The Roman invasion of Scotland in the later 1^st century AD saw the introduction of significant quantities of glass. However, since the Roman presence here comprised only three short phases totalling no more than fifty years, the degree of Romanization was much less than in England.

Consequently, Roman glass found in Scotland tends to be utilitarian and generally restricted to military Installations, forts, marching camps etc.

This lack of lasting influence probably extended into the Dark Age or Early Historic period and glass remained a rather scarce commodity.

It is with the start of the building of the great abbeys and cathedrals in the 12^th century that glass begins to be used in greater quantities in Scotland. It is at this point however, that mention must be made of a problem with glass, particularly in a buried environment, its durability.

Wine bottle seal from Fetternear, Aberdeenshire: Several seals of this design turned up during the excavations there. The arms are of the Leslies of Balquhainwith the seven balls on the coronet denoting the rank of Viscount.

Unlike pottery which remains largely unaltered in the buried environment, glass can and does decay or denature. The rate at which this happens is determined by two factors, the composition of the glass and the nature of its buried environment.

To understand this it is necessary to take a quick look at the basic chemistry of glass.

In simple terms glass consists of three basic constituents, silica, a fluxing alkali and a stabilizer. The silica, usually sand, would vitrify on its own but at a very high temperature, over 2000 degrees C. However, achieving that sort of temperature would demand huge amounts of energy and the resulting glass would have to be maintained at very high temperatures to remain workable, clearly impractical. A fluxing alkali is therefore added to the batch which lowers the temperature of vitrification to c1200 degrees C and, equally important, the temperature at which the glass remains workable.

Unfortunately this silica/alkali mix tends to be unstable and a stabilizer, usually lime, is added. When glass lies buried, particularly in damp soils, the stabilising lime can begin to leach out and the glass begins to break down.

Again in simple terms the fluxing alkali is either potash (potassium based) or soda (sodium based). For reasons that are still not fully understood, potash fluxed glass is generally more prone to denaturing than soda fluxed.

Small late 17th to early 18th century medicine bottle from Fetternear.

The majority of glass used in Scotland in the medieval period was potash fluxed. This is particularly true of window glass which is often found in very poor condition, indeed a significant proportion has virtually disappeared. Scottish soils are typically acidic except in areas where younger limestone rocks make up the local geology. Even potash fluxed glass survives quite well in acidic soils. However, since most glass is found in or around the remains of built structures the pH of the local environment can be quite different. The presence of lime mortar creates an alkaline environment and the effect on glass can be drastic.

While denaturing is obviously a major problem in the preservation of glass artefacts, for the archaeologist it is not necessarily all bad news. Quite often clues to the chemistry of the glass can be derived from the denaturing. For instance potash-fluxed window glass denatures differently from soda-fluxed. With the former the denaturing can be very intrusive and quite uneven whereas the latter tends to occur in discrete layers leaving the underlying heart glass unaffected. The basic chemistry of the manufacture of window glass was changed over time and denaturing can sometimes assist in rough dating.

One of the most useful artefacts of the Post-Medieval period is the glass wine bottle. These started to be made in England about 1630 and in Scotland by 1660 at the latest. They were made from potash-fluxed glass and denature readily. Slight differences in the batch chemistry lead to differences in the appearance of the denaturing. Wine bottle shards turn up in great quantities on Scottish sites, particularly in 18^th century contexts. Quite often substantial pieces of neck and lip or bases turn up and these can be matched through their denaturing.

Wine bottles underwent a radical shape evolution from their introduction c1630 through to the advent of whole body moulding in the 1820s. This has enabled a good date by shape typology to be developed.

Wine bottle from Alloa Tower: Typical late 18th century shape probably made at the Alloa glassworks.

Since we have no evidence, as yet, of indigenous glass manufacture from raw materials in Scotland before c1610 then we must assume everything before that was imported.

My study of glass from a whole host of Scottish sites over the last 30 years has identified many and varied vessels from Holland, Belgium, France, Germany, Italy and Bohemia, even probably Syria in one case. This latter took the form of three small shards of Islamic glass from a probable 13^th century Syrian beaker. They were found at Old Caerlaverock Castle (1220-1270), Dumfriesshire, and as far as this author is aware, is the only Islamic glass so far found in Scotland. Apparently one of the Maxwells of Caerlaverock was on the 7^th Crusade!

Glassworks in Scotland were relatively few until the 19^th century and only two, as far as I’m aware, have been the subject of formal archaeology.

The Verreville (literally glass town!) works at Finnieston St, Glasgow, were excavated in 2005. This glassworks was set up in 1777 and glassware produced until 1842. The site had been partially converted to a pottery in 1834 and was given over completely to that activity in 1842. In the 1920s a large steel framed industrial building was superimposed on that. The latter had huge concrete foundation pads which destroyed a considerable amount of the underlying archaeology. The usual constriction on time ahead of redevelopment further exacerbated problems and little of the glassworks could be positively identified.

A community project at Prestonpans has seen discovery of part of a glassworks at Morison’s Haven, formerly a small harbour where reclaimed land now prevails. Historical reference to this site dates as far back as 1625 but the phrasing of the reference indicates it was already an existing concern at that date. Unfortunately very little survived apart from a section of a flue, air intake for the furnace, which appears to have seen later service as an air raid shelter.

Probably the most promising potential site is that of the later Leith Glassworks, situated on the N side of Salamander Street. Originally set up in 1662 in the Cromwellian Citadel of Leith, glassmaking moved to another North Leith site, probably bounding the river, before moving to the then sands of South Leith in 1747.

The glassworks at South Leith would ultimately comprise an Edinburgh company with 4 cones (glasshouses), a Leith company with 3 cones and a Crystal works, all ranged along Salamander Street. The Edinburgh and Leith companies amalgamated in the 1820s.

Glassmaking at this site continued till c1875 when the works were advertised for sale. Activity on the site thereafter seems to have been restricted to large industrial units, eg a chemical works in which some of the cones seem to have been put to another use judging by large scale map evidence. The Edinburgh company’s site is presently occupied by Keyline Building Supplies and there are bits and pieces of glassworks era structure still surviving.

Although the superstructure of the last cones on the Edinburgh company site were demolished c1912, it is very likely that a considerable amount of the below ground infrastructure survives. Given the rate of redevelopment at Leith, it is probably only a matter of time before this site becomes available for examination.

I await this with considerable interest!

		K Robin Murdoch November 2010             all images copyright ©2010 K. R. Murdoch
	Fast Castle finds:  Drawings of vessel glass finds from Fast Castle, Berwickshire, 15th/16th century.

1829 map showing the glassworks at Leith at their fullest extent, a total of seven cones plus a crystal works.