History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c16.s6: Colouring Agents
History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c16.s6
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Colouring Agents
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(i) From the Harappan period onwards copper (both in cupric and cuprous form) has been used to obtain various shades of blue, green and opaque red in conjunction with iron.
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(ii) Use of manganese is restricted and has been reported from glasses from Kausambi (1. 42%) and Taxila glasses have upto 0.34%. Glasses of violet colour do not seem to be of indigenous origin. Such glasses as reported from Arikamedu and Ter look to be of Roman origin. Use of 0.10% MnO in glasses containing 2.5% Fe2O3 is reported for making colourless glass at Kopia.
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(iii) Use of Cobalt : Its earliest use in India is attested in glass beads from Rajghat datable to early centuries of Christian era (1st-3rd century A.D.) and subsequently cobalt is also reported from glasses from Mason. The percentage reported varies from 0.01 to 1.90%.
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(iv) Colour mechanism of copper-red glasses of Indian origin : Indigenous Cu-red glasses are free from lead and have been coloured red with substantial addition of copper and iron oxides (Bhardwaj 1970).
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(a) The indigenous copper - red glasses reported from Rajghat, Kausambi, Nalanda, Ahicchatra etc. show complete absence of lead in contrast to Assyrian red glasses which have high content of lead (about 22%).
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(b) another peculiar feature of Indian copper-red glasses is high content of iron oxide (9.67% in Rajghat, 9.82+7.01 in Nalanda and 9.16 in Assam glasses).
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(c) Most of the Indian copper-red glasses have very high percentage of cuprous oxide e.g.: 5.27% in Rajghat, 9.13% in Assam and 10.89% in Kausambi.
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(d) In contrast, glasses in other parts of the Old World have in general low content of iron oxide which is generally limited from 0.75 to 3.26%.
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(e) However, cuprous oxide in very early copper-red glasses e.g.. XVIIIth dynasty Egyptian 1400 B.C., Tel-el-Amarna, and 8th-7th century B.C. Assyrian glass from Nirmud have 13% cuprous oxide.
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Foreign Elements in Indian Glasses
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By and large, glass industry in ancient India has been indigenous and limited to making glass beads and bangles. This limitation has been due to short supply of alkalies.
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Beads look to have been exchanged or brought from one country to another and in this light we find quite a number of glass beads which look to be of foreign origin. In this connection mention may be made of:
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1. Taxila (now in Pakistan) was once a province of Achaemanian empire, conquered by Cyrus and it is very likely that quite a few glass objects found from Taxila excavations are of foreign origin. Presence of substantial amount of lead in some of these glasses is a pointer in this direction. Use of antimony and tin also confirm this (Bhardwaj 1979).
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2. Potash glasses with high content of silica have been reported from Ter, Kausambi, Arikamedu etc and these also look to be of foreign origin - probably Roman.
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3. Tripuri has yielded 3 glass beads (datable to circa 200 B.C.- 200 A.D.) the chemical composition of these beads shows the occurrence of barium in conjunction with lead. This is a diagnostic combination of Chinese glasses of the Han Period. This strongly suggests that these beads are of Chinese origin and attest to Indian and Chinese contacts around 200 B.C.-200 A.D. (Bhardwaj 1984).
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Dating value of Glass
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The question is whether it is possible to pin point any characteristic features so as to date the glass finds of archaeological origin. The answer is that there is some potentiality: provided glass beads from various horizons of archaeological excavations are analysed and diagnostic features underlined. At present state of our knowledge, a the following features can help in dating:
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(i) Iron is the only colouring agent in P.G.W. levels. Chromium has been recently reported from the same levels but this is a solitary evidence.
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(ii) Copper (both Cu+ and Cu++) is the main colouring agent along with iron which is always present, mostly as an impurity of the raw materials.
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(iii) Use of manganese both as decolouriser and as a tinctorial agent starts in the beginning of Christian era.
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(iv) Use of cobalt is first noted from archaeological levels datable to 1st to 3rd century A.D.
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So on the basis of their presence or absence one may be able to date the archaeological levels.
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Methods of Fabrication
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A variety of Glass objects e.g. beads, bangles, seals, tiles, bowls and flasks have been reported from archaeological remains. Of these beads are the commonest.
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Fig.7. Simple-wound bead (from Dikshit, 1969). (Courtesy Dikshit, 'History of Glass'.)
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Beads are named differently on the basis of their fabrication and design e.g. wire wound, multiple wound, drawn out beads, cane glass beads, beads made on spoke or hand perforated beads, pressed beads, folded beads and decorative beads like eye beads, gold foil beads, flowered beads (millifiori beads) and composite beads etc.
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Fig.8. Multiple-wound bead (from Sen et al., reproduced from monograph on Glass, by Sen, INSA).
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Bead Making: The glass in molten condition was stirred with an iron bar and the mass of molten glass is gathered at its end. Another craftsman pulls out this gathered mass by fixing another iron bar to it. He pulls it quickly by running, resulting in the formation of long glass rods with average thickness of 15 mm. Glass tubes were also made similarly but the gathering was provided with an air cavity and tubes were drawn out from the gathering with a blow iron. To start with beads would have been made singly by hand by winding thin glass threads round a copper wire and breaking off the thread after each bead (Fig 7). Beads from glass tubes were made by drawing out the glass tube to the required diameter and then cutting it into beads.
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Having made the glass rods and canes and tubes of the requisite size the beads were made by winding the softened glass rod around wire, the thickness of which varied according to the desired perforation of the bead. Beads were made either by single or multiple winding and they were fire polished and were turned around the fire to give the desired shape (Fig 8).
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On cooling the beads were taken off from the wire without any difficulty due to greater contraction in the thickness of the wire. Multiple cane beads were fabricated by pulling out bundle of glass canes together and giving them desired shape (Fig 9). Uniform surface was obtained by heat treatment. Beads were also made on spoke or were obtained by perforating semi-molten dropping of glass. Special beads like eye bead were made by highly skilled worker by spreading layers of different glasses on a matrix resembling the shape of eye. There is a design of eye surrounded by white matrix. Gold foil beads were made by sandwiching a gold foil in between two semi molten halves of the glass bead. Composite beads were made by adding an intermediate layer of white glass in between coloured glasses and then moulding them by twisting.
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Bangle Making: In the simplest method the craftsman dips the aṅkuri (iron hook) into the molten glass and takes out each time a small ball of glass sufficient for a single bangle. This ball is wound up on the end of an iron spit giving it the form of a thick ring which is squeezed with the help of a dagger shaped tool known as a mallahe - till it is cool. The ring is detached with the help of iron wire (bandhana) and is transferred to tapering clay cone (kalbut) which is processed in the furnace and spun rapidly and the glass ring is moved on to enlarge to the required size and is then slipped off and cooled.
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Fig.9. Drawn beads (from Sen et al., reproduced from monograph on Glass, by Sen, INSA).
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*Making of Hollow Wares by Core Techniques*: Hollow wares as evidenced from Devnimori, Amreli, Ter etc look to have been made by core technique. A stick or a rod with conical end was coated with a ball of sand held together by cloth bag. This was either dipped into molten glass and immediately taken out or thin canes of glass in semi-molten condition were wound around the core and fire polished. In both the cases, the core was removed on cooling and hollow ware was produced.
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*Moulding of Glass Objects*: As in the case of metal casting some glass objects were also prepared by casting in clay, sand stone or siliceous moulds. The molten glass was poured into the mould. Glass tiles, seals, and some bangles were made by casting them in moulds.
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*Glass Blowing Process*: Though glass blown objects like glass flasks from Taxila are reported but these look to be imports. Blown glass objects seem to have originated around first century B.C. in Rome. Blowing was done either in a mould or free blowing. In mould blowing, gathering of required size was taken on the blow iron and blown a bit and introduced into the bivalve mould and then blown to fill the mould. In case of free blowing a bulb of glass was blown and shaped by marvering or with the help of a handled board (*Fig 10*).