Previous work

3.3.1 Chemical analysis

Many of the sites detailed in this study have been investigated previously by a variety of scientific means. Glass from Buckholt, Buckholt West and Kimmeridge has not been previously analysed. The following table summarises the previously published chemical analyses carried out on samples from the sites investigated in this project:

Site Date Technique No. of

SEM-EDX: Major, Minor 50 Merchant et al. 1997;

32 Knightons Mid-16th

century

SEM-EDX: Major, Minor 15 Green and Hart 1987;

280

Unknown: Major, Minor 3 Bridgewater 1963; 310

Bickerstaffe Early-17th century

SEM-EDX: Major, Minor 6 Vose 1995; 16, 20

Kimmeridge c.1617-1623 N/A N/A N/A

Haughton Green c.1615-1653 Unknown: Major, Minor, Trace

5 Vose 1994; 57 Table 3.2: Sites studied in this project with details of previously published analyses

(N/A=no published analyses found).

The techniques used, number and types of samples analysed, and age of the studies varies greatly and therefore comparing these data will be problematic. None of the studies give any type of glass standard analysis data and so it is not possible to compare the techniques used. Separately the results, especially those from the more recent studies, may be considered useful. However, direct comparison between

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studies must be carried out with care. The data for some oxides is presented in different forms, for example in Crossley and Aberg’s 1972 report on the excavations of Rosedale and Hutton Al2O3 is combined with TiO2

and presented as a combined percentage. Problems such as these have led to the selection of only a few significant major elements for comparison in this study. These are presented in table 3.3 below.

The sites are presented in order of date, see table 3.2 above for dates.

Na₂O MgO SiO₂ K₂O CaO total alkali

Table 3.3: Average values for a selection of oxides for previously analysed glasses from sites studied in this project (see table 3.2 for publication details).

The compositions vary greatly between sites. The main differences between the glasses from these sites are the alkali and lime levels. Earlier sites have more potash, and later sites more soda. This is a very approximate trend and in the case of some sites the results are based on very few samples. Blunden’s Wood, Bagot’s Park and Little Birches have the lowest calcium oxide levels, and the highest levels are found at the later sites. The glasses analysed from Bickerstaffe and Haughton Green have the lowest potash levels and highest soda levels. This compositional change was discussed in chapter 1 and is linked to a switch to the use of coal as fuel, necessitating the use of a different, apparently more soda-rich ash, rather than any of the wood ash produced in the furnace.

Very approximately the glasses fall into four compositional groups:

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1. High potash, low calcium oxide: Blunden’s Wood, Bagot’s Park, Little Birches

2. High potash, high calcium oxide: Knightons, Glasshouse Farm 3. Mixed alkali, high calcium oxide: Hutton and Rosedale

4. Higher soda, high calcium oxide: Bickerstaffe and Haughton Green

These four groups are approximately chronologically ordered. Group 1 is 14th century to mid-16th century, Group 2 is mid- to late-16th century, Group 3 is late 16th century and Group 4 is early- to mid- 17th century. There may also be some geographical ordering of the Groups as, apart from Knightons and Blunden’s Wood, where any two sites are located in the same region of the UK they fall into the same compositional group.

These compositional trends have been noted for glass produced in England and abroad by other authors (Mortimer 1997; Hartmann 1994) and are discussed in terms of raw material uses in chapter 1. The small numbers of samples analysed do not allow these patterns to be confirmed with any confidence for the sites in this study.

However, the results do suggest that they fit in with these previous trends.

Summary

These results, while limited in number, have shown that there is some compositional patterning which may be related to both the chronological and geographical location of the production sites. Compositional differences, which were expected from changes in raw materials at later sites, can be observed. In no case are there sufficient compositional differences to suggest a site-specific composition has been found. The patterning seen is still very useful and suggests that recipes altered over time. Also, glasshouses within a single region may have used a similar recipe showing that there may have been some communication between sites.

It must be noted here again that these conclusions are based on very limited sample numbers analysed by varying analytical techniques. The results of the analyses carried

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out in this study will be able shed more light on these emerging patterns. They will also allow more confidence to be given to conclusions as the analyses were all carried out using the same methodology.

3.3.2 Isotopic analysis

Only a very limited amount of isotopic analysis has been carried out on forest glasses, and none has been carried out previously on the samples from the sites studied in this thesis. Using strontium isotope ratio and concentration data Dungworth et al. (2009) (discussed in chapter 1) showed that seaweed was used in the production of forest glass at the site of Silkstone, located over 100km away from the coast. They also suggest that other alkali sources used at this site may have been grown in geologically very old regions of the UK, which only exist at least 130km from this site.

This evidence challenges the idea that many of the raw materials used in forest glass production were sourced locally. The use of seaweed, traded over such a long distance, suggests that it was in demand and that glassmakers must have considered it a very useful raw material. Their conclusions show that the isotopic analysis of glasses such as these can shed light on the types of raw materials used, and potentially a very approximate idea of possible source locations.