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Methods of glaze application
sprinkling or 'dusting'
Sprinkling lead in any form onto a wet pot without a binder is generally unsatisfactory, although the application of glaze in this way to some medieval pots cannot be ruled out. As the vessel dries and contracts much of the lead falls away and is easily dislodged in handling. The use of a clay slip or flour binder improves adhesion, as does the use of finely divided particles. Nevertheless sprinkling methods fall well short of providing a uniform and controllable glaze. There is little convincing evidence to connect the absence of glaze underneath handles with the use of sprinkling.
One of the main problems of sprinkling methods is the failure of much of the lead to become integrated with the binder and in consequence falling off during firing. This is easily overcome by adding the lead to the binder before applying it, either by adding flour or mixing it with ground dry clay before adding water. The glaze-binder mixture can then be applied by brushing or pouring, and this simple change will yield a major gain in efficiency.
brushing
Surface adhesion is improved by brushing-on the combined lead and binder, perhaps because of the scoring action of the brush which also permits a stiffer and less water-saturated mixture. Fine particles of lead will usually produce a better glaze than coarse ones.
Many medieval examples of so-called 'splash glazing' are likely to have been brushed or poured instead. Even when the glaze mixture is brushed on carefully and gives the appearance of a uniform application metallic lead or coarse galena is unlikely to be evenly distributed on the surface of a vessel. The heavy particles tend to collect or clump together and form numerous small patches of lead as well as larger unevenly applied areas. When viewed on the fired pot these clumps and depositions of lead take on the aspect of splashes, and the glaze may erroneously appear to have been produced by splashing it onto the pot, or by sprinkling the ingredients onto the vessel's surface.
An effective alternative to brushing is the use of a generous handful of sheep's wool dipped in glaze and applied to the pot. The oil in the wool prevents the glaze from saturating the material and allows it to spread easily onto the surface. Whether some medieval pots were glazed in this way is of course an open question, yet the method is so successful, and loose wool so abundant, that it seems likely.
pouring and immersion
Pouring the glaze over the exterior of a vessel is a rapid and effective way of achieving a uniform cover.This can be done by gripping the inverted pot at the base and giving it a twisting movement while letting the glaze run over the surface from a basin. Pottery poured in this way may have characteristic scallop-shaped edges to the basal glaze boundary. The binder must be fluid enough to pour easily and give an even coat of glaze, consequently the glaze ingredients must remain suspended in the liquid. Dust-ground galena, litharge and lead carbonate will generally stay in suspension, although coarser forms of lead will quickly fall out. These lead particles tend to clump together when poured and, as in the case of brushing, a fired vessel poured with coarse ingredients may have the appearance of being splashed or sprinkled instead.
Glazing may also be done by immersing the leather-hard vessel in a container of liquid glaze. Upright immersion results in a glaze cover on the base bottom which wiping is unlikely to remove completely. Inverted immersion enables trapped air inside the pot to prevent full glaze coverage of the interior, although some glaze will appear around the inside of the neck. It will give a complete glaze cover beneath handles.
Glaze colour
Generally speaking, in an oxidising atmosphere the glazed surfaces of iron-bearing pottery fire to various shades of yellow or brown, and reduced glazes form a family of olive-green or brown-green colours. The particular colour, or range of colours, of an oxidised or reduced glaze will depend upon the character of the clay used and especially its iron content, the composition of the glaze, and the technique used in firing and placing the ware in the kiln.
reduction
During a reduction firing, lead oxide combines with silica naturally present in the iron-bearing clay to form a glaze melt in which reduced black iron oxide is an actively involved flux. Once any form of iron oxide is involved in vitrification it is difficult to change its properties by altering the kiln atmosphere. To produce a typical medieval olive-green reduced glaze ferric (red)oxide must be reduced to ferrous (black) oxide before vitrification, consequently the kiln must be reducing before the glaze begins to melt, at least from 600C. Glaze fired in a reducing atmosphere can be changed to oxidised colours by introducing a generous supply of air into the kiln before the iron oxide is locked into the glaze melt. Oxidation after the reduced iron has entered the melt in fusion with silicates is unlikely to bring about a substantial change of colour.
copper green oxidised glazes
The addition of copper to a glaze in a largely oxidised firing will produce a deep, sometimes brilliant green colour. The copper can be applied in the form of fine particles filed from a piece of copper metal, or in the form of copper carbonate. Copper filings will usually produce a characteristically speckled-green medieval glaze.
Glaze quantities
Variations in materials, working methods and kiln practice make it difficult to estimate the quantity of lead medieval potters needed to glaze a jug of a specific size. On the basis of the experimental production of replica jugs, it can be said that a mixture by dry weight of three parts lead and one part flour, or 10-20% clay, represents a safe recipe for both oxidation and reduction with any form of lead. Nevertheless a perfectly good glaze may result from either larger or smaller quantities of lead.
The quantity of copper needed is about 1-2g for a 30cm high jug. Copper much in excess of this in a lead glaze is likely to crystallize out upon cooling and give the surface a dark, somewhat metallic finish. The quality of the glaze depends almost entirely on how finely the copper is divided and the form in which it is introduced into the glaze.
Last page of glaze
Further reading: Newell, R.W., Some Notes on 'Splashed Glazes', Medieval
Ceramics, 19, 1995
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