The composition of the soda-rich and mixed alkali plant ashes used in the production of glass
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Journal of Archaeological Science 33 (2006) 1284e1292
http://www.elsevier.com/locate/jas
The composition of the soda-rich and mixed alkali plant ashes used
in the production of glass
M.S. Tite a,*, A. Shortland b, Y. Maniatis c, D. Kavoussanaki c, S.A. Harris d
a
Research Laboratory for Archaeology and the History of Art, Oxford University, 6 Keble Road, Oxford OX1 3QJ, UK
b
Centre for Archaeological and Forensic Analysis, Department of Materials and Medical Sciences, Cranfield University, Shrivenham,
Wiltshire SN6 8LA, UK
c
Laboratory for Archaeometry, Institute of Materials, NCSR ‘‘Demokritos’’, 15310 Ag. Paraskevi, Attikis, Greece
d
Department of Plant Sciences, South Parks Road, Oxford OX1 3RB, UK
Received 26 August 2005; received in revised form 19 December 2005; accepted 9 January 2006
Abstract
Soda-rich plant ashes have been used in the Near East and Egypt in the production of glass and faience from the 4th millennium BC onwards,
and mixed alkali plant ashes have been similarly used in western Europe during the 2nd and first half of the 1st millennia BC. In the production
of these ashes, the plants of interest are salt resistant, halophytic plants of the Chenopodiaceae family, growing in coastal, salt marsh and desert
regions. A primary criterion in selecting ashes for glass and faience production is that the alkalis are predominantly in the form of carbonates,
bicarbonates and hydroxides rather than either chlorides or sulphates. In the current paper, previously published data for such ashes are brought
together and re-assessed, and new analytical data are presented for ashes produced from plants collected in Egypt, Greece and the UK. For the
ashes produced from Salsola kali plants collected from Greece and the UK, the soda to potash ratios (0.3e1.8) do not show any systematic
differences between the regions in which the plant was growing, but instead reflect the fact that this species favours the accumulation of Kþ
over Naþ ions. Further, the results suggest that S. kali could have been the source of the mixed alkali ashes used in western Europe, if the ashes
had first been treated in some way in order to reduce their lime-plus-magnesia contents.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Plant ash; Halophytic plants; Egypt; Near East; Greece; UK; Faience; Glass; Bronze Age; Chemical analyses
1. Introduction usually a little higher than those of soda, and by low lime and
magnesia contents. Subsequently, during the later Bronze Age
In the Near East and Egypt, soda-rich ashes derived from (from about 11th to 8th century BC), a similar mixed alkali
halophytic plants were used as the flux in the production of plant ash was used in the production of glass that is found
glazes on small objects made from quartz, steatite and faience throughout Western Europe, with Frattesina in northern Italy
from the 4th millennium BC, and for the production of glass probably being a major centre for its production [5,2].
from about 1500 BC onwards [12]. Also during the 2nd millen- Around the beginning of the 1st millennium BC, soda-rich
nium BC, there is evidence that a mixed alkali plant ash was plant ashes were replaced by the natural evaporite, natron,
used in the production of faience and Egyptian blue in the from the Wadi Natrun in Egypt as the flux used in glass
Crete [17], and in the production of faience in western Europe production in the Levant and Egypt. Subsequently, by the
[11]. This plant ash is characterised by potash contents that are 5th century BC, natron was the flux used in the great majority
of glass produced west of the Euphrates, and fed the prodi-
gious growth of glass production during the Roman period
* Corresponding author. Fax: þ44 1865 273 932. when natron-based glass spread throughout Europe [12]. Glass
E-mail address: michael.tite@rlaha.ox.ac.uk (M.S. Tite). production across the Levant, the Mediterranean and Europe
0305-4403/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jas.2006.01.004M.S. Tite et al. / Journal of Archaeological Science 33 (2006) 1284e1292 1285 continued to be based on natron until around the 9th century for both glass and soap production is relevant when selecting AD when the pressure on its supply appears to have become plant ash analyses for consideration. such that it ceased to be used as the flux in glass production The factors that determine the compositions of plant ash in- in the Islamic Near East [6]. Here, natron was replaced by clude the plant species; the stage in the growing season and the soda-rich plant ash, which had continued to be used as the component of the plants (woody part or leaves); the composi- flux in glass production in Mesopotamia and Iran throughout tion of the soil and ground water in which the plants are grow- the period of natron dominance to the west. At more-or-less ing; and the way in which the plants are ashed [7]. The major the same time, a potash-rich plant ash (forest plant ash) started corpus of analytical data for ashes produced from halophytic to be used consistently as the flux in glass production in west- plants and seaweeds is provided by Brill [3, vol. 2, pp. 482e ern Europe. 486] with additional data by Ashtor and Cevidalli [1], Turner In the context of the soda-rich and mixed alkali plant ashes, [18] and Verita [19]. Analytical data for this particular range the plants of interest are salt resistant, halophytic plants grow- of plant ashes are very limited within the botanical literature. ing in coastal, salt marsh and desert regions, or seaweeds. Salt However, the effect of ashing temperatures on the compositions resistance is a plant’s ability to avoid, by means of salt regula- of the resultant ashes is discussed. Misra et al. [8] present data tion, excessive amounts of salt reaching the living cell, or tol- for the compositions of wood ashes produced at temperatures erating the toxic effects associated with increased ion in the range 500e1300 C, and show that increasing propor- concentration. Multiple mechanisms for salt regulation have tions of potash are progressively lost by volatilisation for ash- been described in the ecological literature, including salt elim- ing temperatures above 800e900 C, depending on the ination through specialised glands and hairs, the shedding of species. Soda appears to be significantly less susceptibility to salt-containing plant parts, succulence and the accumulation volatilisation than potash [15], although no data are available of salt in cell compartments [4]. Of particular importance in for ashes produced from halophytic plants. the present context are the Chenopodiaceae which is a family The aims of the present paper are twofold. The first is to of some 1300 species in 120 genera. The family is found world- bring together and re-assess the previously published data wide, especially in desert and semi-desert regions, and is well for the chemical compositions of soda-rich and mixed alkali known for containing many halophytes. Indeed, many genera plant ashes (Table 1), and the second is to present data for (e.g., Suaeda, Salsola, Salicornia) are obligate halophytes, the chemical compositions of a small group of ashes produced that is they are absent from areas with low soil salinity. in the laboratory from plants collected from Egypt, Greece and In selecting plant ashes suitable for glass production, the pri- the UK (Table 2). The suitability of these different plant ashes mary criterion is that the alkalis (sodium and potassium) should for the production of ancient faience and glass (Table 3) is be predominantly in the form of carbonates, bicarbonates, sul- then briefly considered. phites, sulphides and hydroxides rather than either chlorides or sulphates. This is because the alkali chlorides are practically 2. Experimental procedures non-reactive, melting and volatilising without decomposition. Similarly, the alkali sulphates react only slowly prior to decom- Samples of four different plants species were collected position (occurring at about 1200 C for sodium sulphate) un- from three locations in Egypt in March 2004. Samples of Sal- less reduced to sulphites or sulphides as a result of the presence sola kali were collected from four coastal locations in Attica of carbonaceous matter [18,20]. Therefore, unlike the carbon- and on Crete in June 2004, and from three coastal locations ates which dissociate to form oxides and are then readily incor- in Pembrokeshire and from one on the Isle of Mull in July porated into the glass, only small amounts of chlorides or and August 2004. Details of the plant species and locations sulphates can be incorporated into a glass (typically 1e2% from which they were collected are given in Table 2. With each of Cl and SO3) with the remainder forming an immiscible one exception, all the plants collected were annuals, and there- melt that remains separate from the glass. fore, in each case, only a single sample, consisting of stems, A further use to which plant ashes, as well as natron, were leaves and buds, was taken either from an individual plant put in antiquity was for the production of soap and detergents or from a small clump of plants. The exception was the Suaeda [21, pp. 284, 1]. In this case, the ashes were first treated with shrub from Barnug, Egypt from which separate samples of the lime water (calcium hydroxide) to convert the sodium carbon- woody component (WM2A) and the more fleshy stems ate to sodium hydroxide. Boiling the sodium hydroxide solu- (WM2) were taken. The weight of plant, as collected, was typ- tion with fat or oil produced the soap or detergent. As for ically 50e150 g. glass production, the alkalis in the plant ashes used in soap The plants were dried in air for one to two weeks, the re- and detergent production should be predominantly in the sulting dry weight of plant being typically 5e20 g. The plants form of carbonates since chlorides and sulphates cannot be were subsequently ashed in porcelain crucibles in a muffle fur- converted to hydroxides by treatment with lime water. Further- nace first by heating at 300 C for 1 h and then igniting at more, the nature of the soap produced varies according to 600 C over a period of 10 h, during which time they were whether sodium or potassium is the dominant alkali, a high periodically removed from the furnace and the powder stirred soda ash resulting in soap with a solid consistency, and to ensure complete ignition. The resulting ashes, which were a high potash ash in a more liquid or gelatinous soap. In the white powders with no obvious surviving carbon, were present context, the fact that similar plant ashes were used weighed immediately after cooling in order to minimise
1286
Table 1
Plant ash compositions e published analytical data (%wt oxide)
Numbera Na2O K2O CaO MgO P2O5 CO2 SO3 Cl SiO2 Al2O3 FeO Na2O/K2O CaO þ MgO/ %Carbonated Mole
Na2O þ K2O CO2
ratioe
Near East
Syria e desert Shrub (chinan) 1380 31.3 5.2 9.5 6.0 28.2 8.1 15.0 0.5 0.7 6.0 0.43 44.2 2.5
Syria e desert Ash lump (chinan) Turner 28.0 5.5 21.1 0.5 1.8 34.0 2.2 2.1 5.1 0.64 88.8 1.6
Syria e desert Ash lump (chinan) 1381 24.0 15.3 21.2 11.2 29.0 2.2 3.4 2.0 1.8 1.6 0.82 86.3 1.3
Iraq Shrub (chinan) 1326 42.5 7.0 4.7 12.2 26.4 2.2 6.1 0.5 0.5 0.3 6.1 0.34 85.1 0.9
Iraq Shrub (desert) 1324 25.5 5.0 13.8 8.0 8.6 1.7 2.0 4.9 3.5 5.1 0.72 89.3 0.5
Iraq Shrub (chinan) 4401 24.9 6.8 4.0 7.3 0.1 27.0 1.4 2.3 0.8 0.5 3.7 0.36 89.5 1.4
M.S. Tite et al. / Journal of Archaeological Science 33 (2006) 1284e1292
Iran Ash lump (osnan) 1304 41.2 4.5 3.5 10.0 22.0 0.8 9.0 1.2 0.5 9.2 0.30 80.8 0.8
Iran Ash lump (osnan) 1305 37.3 10.6 6.7 4.0 24.8 1.8 11.4 0.4 0.5 3.5 0.22 74.3 1.0
Pakistan Ash lump 4423 37.7 5.1 4.2 2.9 0.5 28.2 0.7 28.2 0.4 0.4 7.4 0.17 38.6 2.4
Pakistan Ash lump ( ghar)b 4432 28.0 4.8 4.6 4.7 0.8 25.4 0.9 5.1 1.3 0.9 5.9 0.28 83.5 1.3
Pakistan Ash lump ( ghar) 4433 22.3 8.8 7.6 4.0 0.6 15.3 6.3 0.6 1.7 1.4 2.5 0.37 80.8 0.9
Pakistan Shrub ( ghar) 4405 (Rye) 34.3 14.1 3.0 4.3 0.7 21.8 4.0 10.2 0.9 0.3 2.4 0.15 72.4 0.9
Pakistan Ash lump ( ghar-1st) 4420 (Rye) 37.3 9.4 1.6 0.4 1.3 28.8 0.1 8.6 0.2 0.2 4.0 0.04 82.5 1.1
Pakistan Ash lump ( ghar-2nd) 4421 (Rye) 34.1 4.2 4.5 4.2 0.8 23.1 0.4 5.0 1.2 0.4 8.1 0.23 87.4 1.0
Pakistan Ash lump ( ghar-3rd) 4422 (Rye) 30.7 5.9 5.0 5.5 0.7 21.7 0.6 7.4 1.3 0.4 5.2 0.29 80.0 1.1
Afghanistan Ash lump (ishgar) 1331 35.5 4.6 7.3 8.8 23.0 4.7 9.5 1.1 0.4 7.7 0.40 69.0 1.2
Afghanistan Ash lump (tezab) 1330 21.3 17.2 10.8 13.3 1.1 3.8 0.6 0.3 1.2 0.63 87.2
Uzbekistan Ash lump 4447 42.1 7.0 1.0 0.9 0.6 9.8 19.4 15.7 0.2 0.6 6.0 0.04 38.5 0.7
c
Levant Salsola soda (herb) Ashtor 43.0 6.8 3.6 1.8 0.9 3.0 6.4 0.11 93.0
Uzbekistan Salsola crassi 4446 40.3 12.3 1.1 2.6 0.3 23.5 13.1 0.1 0.0 3.3 0.07 38.7
Levant Haloxylon artic (herb) Ashtor 9.8 7.8 22.6 12.6 4.0 0.8 1.3 2.00 74.6
Levant Salsola kali Ashtor 14.3 15.5 14.4 9.0 5.5 1.4 0.9 0.79 77.6
Levant Seaweed 654 14.1 18.1 12.4 10.7 19.8 16.8 19.2 0.6 0.5 0.8 0.72 14.5
Western Europe
Sicily Soda di Catania Verita 23.0 5.7 9.0 2.5 0.6 40.0 1.3 8.0 7.0 1.3 1.3 4.0 0.40 70.1 2.9
Sicily Salsola kali Verita 17.0 9.0 16.0 11.0 2.4 33.0 2.2 0.3 6.8 1.2 1.2 1.9 1.04 91.4 2.1
Venice Salicornia Verita 35.0 4.0 5.0 5.4 0.6 1.2 3.2 37.0 6.0 1.7 0.8 8.8 0.27 7.4 0.6
England Seaweed (kelp) 4520 19.4 16.4 11.6 11.2 0.3 1.5 31.0 5.1 0.7 0.4 1.2 0.64 5.8 1.2
England Seaweed (kelp) 4522 20.8 19.9 9.0 9.0 7.3 1.6 16.0 19.5 0.6 0.4 1.0 0.44 13.2 0.5
England Seaweed (kelp) 4524 18.7 12.4 9.0 8.0 3.1 0.9 23.7 11.4 0.6 0.4 1.5 0.55 5.4
France Ashes of Vareque Verita 19.8 17.5 9.2 6.8 1.0 3.7 20.8 15.5 3.5 0.6 0.4 1.1 0.43 5.3 3.0
Venice Fern Verita 0.4 25.5 18.0 8.0 4.0 25.0 4.5 3.5 8.0 1.5 0.8 0.02 1.00 61.9 3.2
England Bracken Smedley 0.5 35.1 8.9 2.3 3.1 25.0 0.5 0.3 0.01 0.32 85f
a
Numbers refer to data from Brill [3, vol. 2, pp. 482e486], otherwise data are from Turner [18], Ashtor and Cevidalli [1], Verita [19] and Smedley et al. [13].
b
Ghar ¼ khar ¼ ishgar ¼ ashgar ¼ Haloxylon recurvum.
c
Herb refers to plants from herbarium, University of Jerusalem.
d
%Carbonate refers to the percentage of total alkali moles present as carbonate. Negative values indicate that there is insufficient soda-plus-potash present to take up all the Cl2 and SO3 moles.
e
Ratio of analysed moles of CO2 to calculated moles of (Na2O þ K2O) present as carbonates.
f
In this case %carbonate estimated from composition of glass produced using bracken.Table 2
Plant ash compositions e new analytical data (%wt oxides)
Number Na2O K2 O CaO MgO P2O5 SO3 Cl SiO2 Al2O3 FeO Na2O/K2O CaO þ MgO/ %Carbonatea %Ashb
M.S. Tite et al. / Journal of Archaeological Science 33 (2006) 1284e1292
Na2O þ K2O
Egypt e Wadi Natrun Salsola WM1 14.7 3.1 23.1 7.5 0.8 24.8 13.7 8.0 3.0 1.1 4.8 1.72 86.5 27.3
Egypt e Barnug Suaeda WM2 43.4 6.5 5.2 3.9 5.3 8.2 20.0 5.1 1.5 0.9 6.7 0.18 50.1 3.6
Egypt e Barnug Suaeda WM2A (woody) 33.3 14.5 7.3 6.6 4.6 7.1 17.3 7.2 1.4 0.6 2.3 0.29 51.7 17.1
Egypt e Taposiris Magna Anabasis WM3 48.9 4.5 2.0 2.0 2.0 4.2 34.9 0.8 0.5 0.1 10.9 0.07 34.8 18.0
articulata
Egypt e Taposiris Magna Suaeda WM4 30.9 5.3 13.6 4.8 0.7 5.9 32.8 3.6 1.7 0.6 5.8 0.51 3.4 24.8
Attica e Anavissos Salsola kali ANAV1 16.9 31.8 15.3 3.9 2.3 1.5 22.6 4.5 1.1 0.1 0.5 0.39 44.7 16.3
Attica e Schinias Salsola kali SCH1 23.1 19.0 14.4 6.6 1.0 2.1 25.1 7.0 1.1 0.4 1.2 0.50 33.8 22.1
Attica e Schinias Salsola kali SCH2 19.5 26.3 15.7 5.0 1.0 2.2 25.5 3.7 1.0 0.0 0.7 0.45 34.8 20.0
Crete e Ammoudara Salsola kali AMM1 24.8 36.2 11.4 2.2 1.4 2.1 18.7 2.5 0.7 0.0 0.7 0.22 63.1 17.0
Crete e Ammoudara Salsola kali AMM5 19.1 22.2 15.5 6.6 1.3 1.3 31.3 2.0 0.5 0.1 0.9 0.54 15.7 15.8
Crete e Ammoudara Salsola kali AMM5 (pH 7) 6.7 1.5 56.8 26.0 2.0 3.1 0.3 2.4 0.7 0.4 4.6 10.19 64.3 6.5
Crete e Georgioupolis Salsola kali GEOR1 20.0 39.6 10.3 2.4 2.9 1.8 20.9 1.4 0.6 0.1 0.5 0.21 57.3 14.1
Crete e Georgioupolis Salsola kali GEOR4 48.2 26.1 2.1 1.1 1.3 0.9 15.2 3.6 1.4 0.0 1.8 0.04 78.5
Pembroke e Broad Haven Salsola kali BH1 15.2 33.4 13.8 6.2 1.5 2.6 21.4 5.0 0.5 0.2 0.5 0.41 44.2 21.4
Pembroke e Broad Haven Salsola kali BH2 12.3 38.1 10.7 6.9 2.1 1.7 25.1 2.5 0.5 0.1 0.3 0.35 37.7 23.6
Pembroke e Freshwater E Salsola kali FE1 18.1 22.6 16.1 7.2 3.0 2.0 13.9 15.6 0.9 0.5 0.8 0.57 58.4 39.3
Pembroke e Freshwater E Salsola kali FE2 10.6 28.5 23.2 12.5 1.9 3.2 18.6 1.0 0.4 0.1 0.4 0.91 36.2 16.0
Pembroke e Freshwater W Salsola kali FW1 23.7 22.5 11.1 4.1 1.4 2.1 30.6 3.7 0.5 0.3 1.1 0.33 26.1 30.0
Pembroke e Freshwater W Salsola kali FW2 19.2 26.3 9.5 8.4 1.6 2.2 27.0 4.8 0.6 0.3 0.7 0.39 30.6 25.7
Pembroke e Freshwater W Salsola kali FW3 17.5 18.4 14.6 13.0 2.0 3.3 16.9 13.6 0.7 0.2 1.0 0.77 41.5 12.5
Mull Salsola kali IM1 10.9 38.2 19.7 10.2 5.0 2.7 7.5 3.6 1.4 0.7 0.3 0.61 75.9 31.8
a
%Carbonate refers to the percentage of total alkali moles present as carbonate. Negative values indicate that there is insufficient soda-plus-potash present to take up all the Cl2 and SO3 moles.
b
Percentage of ash weight to dry weight of plant.
12871288 M.S. Tite et al. / Journal of Archaeological Science 33 (2006) 1284e1292
Table 3
Glass and faience compositions e published analytical data (%wt oxides)
Locationa Century (BC) Materialb SiO2 Na2O K2 O CaO MgO P2O5 Al2O3 FeO CuO Na2O/K2O CaO þ MgO/
Na2O þ K2O
Near East 14the12th Glass (10) 68.0 15.7 2.3 7.9 3.9 0.6 0.3 1.5 6.9 0.66
Amarna 14th Glass (21) 64.3 17.9 2.3 8.4 4.2 1.1 0.6 1.3 7.8 0.62
Amarna 14th Faience (6) 75.6 7.8 4.2 1.2 0.3 0.4 0.2 10.3 1.9 0.13
Knossos 17th Faience (1) 81.7 4.4 5.1 1.5 0.7 2.9 2.9 0.7 0.9 0.23
Italy 16the14th Faience (4) 68.8 8.1 8.9 4.0 0.4 0.1 0.9 0.7 8.0 0.9 0.26
Frattesina 11the9th Faience (4) 74.6 6.4 10.0 1.7 0.7 0.1 1.7 0.5 4.4 0.6 0.14
Frattesina 11the9th Glass (5) 76.0 6.0 9.9 1.8 0.6 0.1 1.5 0.6 3.5 0.6 0.15
a
Publication of analyses: Near East [3], Amarna [16], Knossos [17], Italy and Frattesina faience [11], Frattesina glass [2].
b
Number of analyses averaged given in parenthesis.
uptake of water or carbon dioxide from the atmosphere. The concentrations in the plant ash do not necessarily provide
weights of ash were in the range 0.5e4 g corresponding to a valid measure of their concentrations in the resulting glass.
ash yields (i.e. percentage of ash weight to dry weight) be- In order to determine the percentages of the total soda and pot-
tween about 4 and 40% (Table 2). ash that are present as carbonates, quantitative X-ray diffrac-
The plants were not washed prior to ashing since, in the ab- tion analyses (XRD) of the ashes, using a suite of closely
sence of recent storms, there would be very little sodium chlo- matched standards, would have been necessary. Only in this
ride on the leaf surface, and for all but minimal washing, salts way would it have been possible to estimate the amounts of
will be removed from within the plant structure. The effect on chlorine fixed as halite (NaCl) and sylvite (KCl), and the
the composition of the ash of extensive prior washing of the amounts of sulphur fixed as thenardite (Na2SO4) and arcanite
plant, such that the water from the final washing was neutral (K2SO4).
(pH 7), is illustrated for the Ammoudara AMM5 (pH 7) sample Instead, on the basis of the chemical analyses reported by
(Table 2). As expected, there was a large reduction in the pot- Turner [19], it is first assumed that the majority of the chlorine
ash, chlorine and, to a lesser extent, the soda contents, and a cor- and sulphur present in plant ash is associated with the sodium
responding large increase in the lime and magnesia contents. and potassium rather than with the calcium or magnesium,
Samples from each of the ashes thus produced were ground the majority of which is present as carbonates. Therefore,
and homogenised, and then pressed into pellets, without the after converting the percent weights of Na2O, K2O, Cl and
addition of any binder, for analysis by SEMeEDS. For each SO3 to moles of Na2O, K2O, Cl2 and SO3, the moles of
pellet, some three different areas up to about 2 mm across (Na2O þ K2O) present as carbonates are taken to be equal to
were analysed, and an average chemical composition deter- the moles of (Na2O þ K2O) minus the moles of (Cl2 þ SO3).
mined. Data were obtained on the chlorine (Cl) and sulphur From this, the percentage of the total moles of (Na2O þ K2O)
(SO3) contents but not the carbon (CO2) content, for which present as carbonates can be calculated (i.e. %carbonate in
a separate analytical technique would have been required. Tables 1 and 2). When all the alkalis are taken up in forming
As a result of porosity and the presence of carbonates, the an- chlorides and sulphates, then the percentage present as carbon-
alytical totals for the pelleted ashes were typically in the range ates is obviously 0%. With increased amounts of alkali
from 60 to 80%. Therefore, for ease of comparison, the analyt- present, some carbonates are present, the amount being
ical data were normalised to 100% totals. For the Egyptian and expressed as a positive percentage in Tables 1 and 2. However,
UK samples, a JEOL JSM-840A with Oxford Instruments Isis if the amount of alkali present is insufficient even to take up all
300 software at the Department of Earth Sciences, University the Cl2 and SO3 moles, then the surviving amounts of Cl2 and
of Oxford was used, and for the Greek samples, a JEOL SO3 moles are expressed as negative percentages in Tables 1
JSM-5600 with Oxford Instruments Inca 303 software at the and 2.
Institute of Materials, NCSR ‘‘Demokritos’’ was used. Both The second major assumption that needs to be made in the
machines were run at 20 kV with a 2e6 nA beam current, absence of quantitative XRD data concerns the distribution of
giving a detection limit for most elements heavier than silicon the Cl2 and SO3 moles between the soda and potash. In the
of around 0.05e0.10 %wt, and 0.10e0.30 %wt for lighter present paper, it is assumed in the interests of simplicity,
elements. Typically, the error on analyses of major elements and thus clarity, that the Cl2 and SO3 moles combine with
is of the order of one percent absolute. the soda and potash in direct proportion to the number of
moles of each alkali present. The advantage of this assumption
3. Interpretation of analytical data is that the total soda to potash weight ratios (Na2O/K2O),
given in Tables 1 and 2, are equal to the corresponding weight
Because, as discussed above, it is primarily the sodium and ratios of soda to potash present as carbonates. Similarly, the
potassium carbonates, and not the chlorides and sulphates, that percentages of the total moles of (Na2O þ K2O) present as car-
are incorporated into the glass, the soda and potash bonates, given in Tables 1 and 2, are equal to the percentagesM.S. Tite et al. / Journal of Archaeological Science 33 (2006) 1284e1292 1289
of the weights of (Na2O þ K2O) present as carbonates. In con- quality (Nos. 4420, 4421 and 4222), the first quality having
trast, if, as assumed by Ashtor and Cevidalli [1], the Cl2 and the greatest porosity and the third containing the highest pro-
SO3 moles are mainly taken up by the soda, the soda to potash portion of charcoal. By heating samples of these ashes in the
ratios present as carbonates will be lower than the correspond- laboratory, Rye and Evans [10, pp. 180e185] showed that
ing total weight ratios, and the percentages of the weights of the first grade ash was the most fusible (i.e. extensive flow
(Na2O þ K2O) present as carbonates will be higher than the by 700e750 C) and the third grade was the least fusible
corresponding percentages of moles. Conversely, if it is as- (i.e. extensive flow by 900e1000 C). This result is consistent
sumed that the Cl2 and SO3 moles are mainly taken up by with the observed progressive decrease in the soda-plus-potash
the potash, the corresponding soda to potash ratios and the per- content (46.7 to 36.6% Na2O þ K2O) and increase in the lime-
centages of the weights of (Na2O þ K2O) present as carbon- plus-magnesia content (2.0 to 16.1% CaO þ MgO) from the
ates will be, respectively, higher and lower. first to the third grade ash.
A further parameter included in Tables 1 and 2 are the The ashes produced from identified species of plants grow-
(CaO þ MgO)/(Na2O þ K2O) ratios which provide a measure ing in the Near East are much more variable both in their soda
of the lime-plus-magnesia contents normalised for variations to potash ratios and in the percentages of alkali present as car-
in the soda-plus-potash contents. bonates. Only the Salsola soda from the Levant matches the
ashes known to have been used in glass and soap production
4. Results in terms of its soda to potash ratio (6.4) and percentage car-
bonate (93%), but even for this ash, its lime-plus-magnesia
The previously published plant ash analyses fall into two content is significantly lower (5.4% as compared to 10.7e
categories. The first group consists of those plants or lumps 32.4%). The soda to potash ratio and percentage carbonate
of plant ash that were acquired from local artisans or pur- for the Salsola crassa from Uzbekistan are both lower (3.3
chased in bazaars, and are known to have been used for glass, and 39%, respectively), and the soda to potash ratios for
glaze or soap production. In this case, plants are referred to by both the Haloxylon articulatum and S. kali are significantly
their local names such as chinan (¼shinan), osnan, ghar lower (1.3 and 0.9, respectively) than the S. soda values. For
(khar ¼ ishgar ¼ ashgar) and tezab. The second group con- the seaweed, the soda to potash ratio for the ash is again close
sists of ashes produced in the laboratory from plant species to 1, but in this case, the alkalis appear to be present entirely as
for which the botanical names are given. Similarly, the associ- chlorides and sulphates rather than carbonates (i.e. negative
ated plant species have been identified for the new ash analy- value for %carbonate).
ses presented in this paper. Both the previously published and The new ashes produced from plants collected from Egypt
the new plant ash analyses are based on single samples. are all soda rich with soda to potash ratios in the range from 2
to 11 (Table 2). However, for the ashes from both the Salsola
4.1. Ashes from the Near East and Egypt species from Wadi Natrun and the Suaeda species from Tapo-
siris Magna, the majority of the alkalis are present as chlorides
The ashes from Syria, Iraq, Iran, Pakistan, Afghanistan and and sulphates, with sulphates predominating for the Salsola
Uzbekistan, known to have been used for glass, glaze and soap from Wadi Natrun. The ashes from the Anabasis articulata
production (i.e. chinan, osnan, ghar and tezab), are all higher from Taposiris Magna and the Suaeda species from Barnug
in soda than potash, the soda to potash ratio varying from both contain significant percentages of carbonates (35e
1.2 to 9.2, with the majority being in the range from 4 to 8 50%), but still less than those for the ashes known to have
(Table 1). In addition the percentages of alkali (i.e. Na2O þ been used in glass and soap production. The principal differ-
K2O moles) estimated as being present as carbonates are con- ences between the ash produced from more woody component
sistently high with the majority being in the range 70 to 90%. (WM2A) of the Suaeda species from Barnug, as compared to
Also, for the majority of the ashes, the ratios of the moles of the more fleshy component (WM2), are a lower soda to potash
CO2 as determined by analysis to moles of (Na2O þ K2O) ratio (2.3 as compared to 6.7), and a higher yield of ash (i.e.
present as carbonates as calculated (i.e. mole CO2 ratio in %ash ¼ 17% as compared to 3.6%).
Table 1) are more-or-less equal to or greater than unity, indi-
cating the presence of sufficient CO2 for the estimated alkali 4.2. Ashes from western Europe and Greece
carbonate content.
Only in the case of the ghar samples from Pakistan col- Only the ‘‘Soda di Catania’’, produced from unspecified Si-
lected by Rye and Evans [10, pp. 180e185] has the associated cilian coastal plants and provided by a present-day glass pro-
plant species, Haloxylon recurvum, been identified. In addition duction works at Murano, Italy, matches in its soda to potash
to bringing back samples of the shrub which were then ashed ratio (4.0), percentage of alkali present as carbonates (70%),
in the laboratory (No. 4405), Rye also brought back ghar sam- and lime-plus-magnesia content (11.5%) the ashes from the
ples produced in Pakistan where, typically, some 400 kg of the Near East known to have been used in glass and soap produc-
shrub were burned over the period of a day in pits some 1.6 to tion (Table 1).
3.0 m in diameter, and dug in the sand to a depth of about The majority of the new ashes produced from S. kali col-
1.6 m. The product of such burnings is friable lumps of ash lected from Attica, Crete, Pembroke and Mull are richer in
of differing porosity that were sorted into three grades of potash than soda, the overall range of soda to potash ratios1290 M.S. Tite et al. / Journal of Archaeological Science 33 (2006) 1284e1292
being from 0.3 to 1.8, with no obvious systematic difference levels vary throughout the growing season of a particular spe-
between these four regions (Table 2). The corresponding range cies across its range.
of percentages of alkali present as carbonates is from 15 to However, even with the present data, it is possible to sug-
80%, but with the majority less than 50%. Thus, these ash gest tentative reasons for some of the more major observed
compositions are comparable with that for S. kali from the Le- differences in plant ash composition. Thus, the high Cl content
vant (Table 1). The ash yields (i.e. %ash) vary between 12 and of Salicornia ash compared to S. soda ash probably reflects the
40%, the higher yields tending to be associated with the ashes difference between a salt marsh and a desert environment. Fur-
produced from plants from Pembroke and Mull. ther, the observation that the potash content of S. kali ash is
For the Salicornia, the soda to potash ratio for the ash is normally greater than its soda content (Na2O/K2O < 1) reflects
again high (8.8), but the majority of the alkalis are present the fact that this species favours the accumulation of Kþ over
as chlorides rather carbonates (Table 1). The seaweed ashes Naþ ions, whether the plants are growing in the Mediterranean
from England (knotted, saw and bladder wracks) and the region or in northern Europe. The range of potash to soda ra-
‘‘Ashes of Vareque’’ are similar to those produced from the tios (0.3e1.8) and percentage carbonates (15e80%) for the
Levant seaweed in that the soda to potash ratios are close to different S. kali samples are then probably the result of varia-
1, but the majority of the alkalis were present as chlorides tions in the relative proportions of sea water, rain water and
and sulphates rather carbonates. In contrast, the ashes pro- run-off from the hinterland that reach the strandline where
duced from fern and bracken, that were one of the sources this species grows, and thus, contribute to the water intake
of plant ash used in glass production in western Europe by the plants. In addition, if marine algae, such as seaweeds,
from the 9th century AD onwards, are potash rich (Na2O/ are actively absorbing salts in order to ensure that water can
K2O ¼ 0.01e0.02), and the majority of the alkalis were again be obtained, then it might be expected that their Na2O/K2O ra-
present as carbonates (62e85%). tios would be low and their sulphate concentrations high.
However, one might also expect that, for coastal rather than
5. Discussion open sea algae, these values would oscillate quite considerably
during a day due to the effects of dehydration.
In order to try to understand the differences in plant ash
compositions observed above, we need to consider both the 5.1. Implications for glass production
way in which halophytes respond to a saline environment, to-
gether with the possible variations in the salinity of this envi- In assessing whether a plant ash would be suitable for the
ronment. Halophytes compensate for water relation problems production of ancient faience and glass, the three primary pa-
in the saline environment by accumulating salt (or organic rameters that need to be considered are the soda to potash ra-
acids and soluble carbohydrates), so that water can move tios (Na2O/K2O), the normalised lime-plus-magnesia contents
into the plant. Similarly, marine algae actively absorb salt so ((CaO þ MgO)/(Na2O þ K2O)), and the percentages of alkali
that water moves into the plant. Ion ratios are often specific present as carbonates (Tables 1 and 2). Average compositions
to particular families or species [7], and most flowering plants together with the values for the first two of these parameters
accumulate more Naþ than Kþ. However, different species are given in Table 3 for faience and glass from the Near
within a genus may respond differently with respect to ion ac- East, Egypt (Amarna), Crete (Knossos) and Italy (including
cumulation: S. kali Kþ > Naþ, Cl > SO2 4 ; Salsola turcman- Frattesina).
ica Naþ > Kþ, Cl > SO2 4 ; Salsola rigida Naþ > Kþ, The 14the12th century BC glass from the Near East and
2
SO4 > Cl [7]. Egypt is characterised by high soda to potash ratios (7e8)
Plants coping with a saline environment must respond to and high lime-plus-magnesia contents ((CaO þ MgO)/
a spatially and temporally, highly dynamic environment. (Na2O þ K2O) ¼ 0.62e0.66). Therefore, it is the lumps of
Only in the open oceans do salt concentrations remain con- plant ash (i.e. chinan and osnan), acquired in Syria, Iraq and
stant (ca. 480 mM Naþ). In the coastal intertidal zone occu- Iran, and known to have been used in glass or soap production,
pied by marine algae, salinity fluctuates over a wide range that provide the closest match (typically Na2O/K2O ¼ 4e9
(e.g., 290e1000 mM Naþ), and thus salts are absorbed or ex- and (CaO þ MgO)/(Na2O þ K2O) ¼ 0.30e0.70) as well as
creted by the algae according to need. Such fluctuations may having high percentages of alkali present as carbonates. The
be diurnal (associated with the tides), sporadic (associated S. soda and Suaeda ashes from the Levant and Egypt
with environmental factors, such as precipitation) or topo- (Barnug), respectively, provide a reasonable match for the
graphic (the position of a plant in relation to rainfall patterns soda to potash ratios (w6.5) but their lime-plus-magnesia con-
and ground and surface water inputs). These features mean tents are too low ((CaO þ MgO)/(Na2O þ K2O) ¼ 0.11e0.18).
that the analysis of salt fluxes through an ecosystem is com- For all the other ashes for which data are available, including
plex and that the expected patterns of salt content within an those produced from S. kali, Salicornia and seaweed, either the
individual plant are hard to predict. Thus, single measurements soda to potash ratios or the percentages of alkali present as
of salt concentration in a single individual from a single pop- carbonates are too low for them to have been used to produce
ulation of a species are of somewhat limited value. Of greater the Near Eastern and Egyptian glass.
value is the analysis of populations from different seasons of In contrast, the mixed alkali faience from Crete and Italy as
the year, in order to build-up a picture of how Naþ and Kþ well as the later mixed alkali faience and glass from FrattesinaM.S. Tite et al. / Journal of Archaeological Science 33 (2006) 1284e1292 1291
exhibit low soda to potash ratios (0.5e1), with the potash S. soda or Suaeda samples have been analysed that match,
content normally greater than the soda content, and low in terms of both their soda to potash ratios and their lime-
lime-plus-magnesia contents ((CaO þ MgO)/(Na2O þ K2O) ¼ plus-magnesia contents, the lumps of plant ash (i.e. chinan
0.14e0.26). As first suggested by Santopadre and Verita [11] and osnan) acquired in Syria, Iraq and Iran, and known to
in the context on the Italian faience and glass, S. kali is a possible have been used in glass or soap production.
source of the plant ash for this entire group of material as far as Therefore, in order to acquire a better understanding of the
its soda to potash ratios (typically 0.5e1.2) are concerned. range of plant species and environments that result in soda-
S. kali is also a suitable plant ash for faience and glass production rich and mixed alkali ashes in which carbonates dominate
in that the percentages of alkali present as carbonates are over chlorides and sulphates, an extended and systematic
normally at least 30e50%. However, the lime-plus-magnesia range of plants, for subsequent ashing and analysis, needs to
contents for S. kali ((CaO þ MgO)/(Na2O þ K2O) typically in be collected from the Near East, Egypt and the Mediterranean
the range 0.2e1) are significantly higher than those observed region. The plants collected should include members of the
in the mixed alkali faience and glasses. Therefore, as again first Chenopodiaceae, e.g., Salsola and Suaeda species, together
suggested by Santopadre and Verita [11], the S. kali would have with the collection of specimens for permanent deposition in
had to have been purified prior to use. This would have involved herbaria, and rigorous species identification. Rather than col-
dissolving the ashes in water, filtering out the insoluble residue, lecting a single specimen of a single population at a single sea-
and recovering the soluble salts by evaporation. In this way, son of the year, the plant collection should be such as to make
a large part of the relatively insoluble calcium and magnesium it possible to obtain some measure of the variation in the com-
compounds would have been removed, and the lime- position of the ash associated with a plant population over its
plus-magnesia content thus decreased. However, it is worth entire growing season. Therefore, as far as it is practical, it
noting that a similar reduction in the lime-plus-magnesia would be necessary to collect samples over a wide area from
content was achieved for the first grade ash ((CaO þ MgO)/ individuals in the population at random, the number being de-
(Na2O þ K2O) ¼ 0.04 as compared to 0.15 for the sample termined by standard approaches to sampling theory, and at
ashed in the laboratory and 0.40 for the third grade ash) least twice in the year.
produced by the bulk firing of ghar plant samples in Pakistan As well as collecting plant samples themselves, samples re-
[10, pp. 180e185]. lating to the environment in which the plants are growing
As previously noted by Tite and Shortland [16], the plant should also be collected. Because most of the plants under
ashes used in the production of Egyptian faience have signif- consideration will be growing in low humus, sandy environ-
icantly lower soda to potash ratios (1.9) and lime-plus-magne- ments, the ion exchange between the plant and its growing me-
sia contents ((CaO þ MgO)/(Na2O þ K2O) ¼ 0.13) than those dium will be low. Therefore, rather than taking samples of the
used in the production of contemporary Egyptian glass. There- growing medium, it is more appropriate to collect samples of
fore, although its soda content is still higher than its potash the ground water. In a desert environment, this is likely to be
content, the plant ash used in the production of Egyptian fa- very difficult e if not impossible e given that the water table
ience appears to be intermediate between the soda-rich plant which the plants are exploiting could be very deep. However,
ash used in the production of the contemporary Egyptian glass, in coastal or salt marsh environments, water samples could be
and the mixed alkali plant ash used in the production of both obtained by digging a hole in the vicinity of the plant and sam-
faience and glass in Crete and Italy. pling the water that collected in this hole. Again, ideally, water
samples should be taken at different points in the growing
6. Conclusions season.
In addition to extending the data on plant ash compositions,
In the context of soda-rich and mixed alkali plant ashes, the the relationship between the compositions of the plant ashes
data for the S. kali ashes are the first to be published for an and those of ancient faience and glass must be further investi-
identified plant species that has been collected over a wide gated. Differences in composition can arise because only small
geographical area. Of particular interest is the fact that ashes amounts of chlorides and sulphates can be incorporated into
produced from plants collected from both the Mediterranean a glass. Also, for faience, there is the possibility of differential
region and northern Europe all exhibit similar soda to potash efflorescence of the glaze components from the body to the
ratios, normalised lime-plus-magnesia contents, and percent- surface during drying, and for glass, the possibility of partial
ages of alkali present as carbonates. Taken together with the batch melting first proposed by Rehren [9] and subsequently
wide availability of S. kali, these new results further reinforce investigated by Shugar and Rehren [14].
the hypothesis that, if first purified by dissolution and subse- Therefore, in order to be able to fully assess the suitability
quent evaporation or treated in some other way to reduce its of a plant ash for faience and glass production, it will be nec-
lime-plus-magnesia content, S. kali could have been the source essary to analyse the plant ashes not only chemically but also
of the plant ash used in the production of mixed alkali faience mineralogically using XRD. However, because of the complex
and glasses. mineralogy of plant ashes and the variable crystallinity of the
Otherwise, however, the results presented above emphasise phases present, fully quantitative XRD will be extremely dif-
the very limited extent of the compositional data for plant ficult. As a result, even with these additional data, it will still
ashes that are currently available. For example, as yet no be difficult to predict with confidence the composition of the1292 M.S. Tite et al. / Journal of Archaeological Science 33 (2006) 1284e1292
faience and glass from that of the plant ash. Therefore, it will [10] O.S. Rye, C. Evans, Traditional Pottery Techniques of Pakistan, Smithso-
also be essential to produce both faience and glass from mix- nian Institution Press, Washington, DC, 1976 (Smithsonian Contributions
to Anthropology No 21).
tures of pure silica and the different plant ashes, and thus com- [11] P. Santopadre, M. Verita, Analyses of production technologies of Italian
pare directly the compositions of the faience glazes and the vitreous materials of the Bronze Age, Journal of Glass Studies 42 (2000)
glasses with those of their constituent plant ashes. Finally, 25e40.
the possibility of reducing the lime-plus-magnesia content of [12] E.V. Sayre, R.W. Smith, Some materials of glass manufacturing in antiq-
S. kali ashes either by purification or by some firing treatment uity, in: M. Levey (Ed.), Archaeological Chemistry, University of Penn-
sylvania Press, Philadelphia, 1967, pp. 279e311.
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Acknowledgements nales du 15e Congres de l’Association Internationale pour l’Histoire du
Verre e New York-Corning 2001, 2003, pp. 203e207.
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