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The active glaze-forming flux in medieval glazes is lead oxide (PbO). The oxide enables a glaze to form at low temperatures by lowering the melting-point of the silica with which it reacts in the body of the pot. The silica and alumina needed to form a lead glaze can usually be derived from the body of the vessel itself.
forms of lead
Lead oxide is introduced into a glaze by the use of one of a small number of lead compounds, which differ in their origins, their application and their behaviour in the kiln. All forms of lead have a practical top temperature limit of about 1150C in oxidation. Little in the way of effective glazing can be accomplished below 780C.
lead metal (Pb)
Ground metallic lead heated on the surface of a pot in contact with air will convert to PbO at dull red heat. A patchy glaze melt will occur before 800C and by 860-920C an adequate glaze melt will be achieved. The melting temperatures will be higher if a clay binder is used. Granulated metallic lead provides a reliable glaze in the temperature range 860-1100C, being at its best around 920-1000C, and can yield a good reduced iron green when fired in reduction on a suitable iron-bearing body.
Galena (PbS)
Galena, or lead sulphide, is the most common ore of lead. Its brittle, silvery crystal structure is easily ground to a powder, making it highly suitable for a glaze. Below 900C a galena glaze is apt to appear underfired, although much will depend on the clay body, the binder and the rate of temperature rise in firing. The most favourable firing range is in the region of 950-1100C.
Litharge (PbO)
Litharge is produced in a reverberatory furnace by exposing melted lead at dull red heat to the action of air and continually skimming the oxidised dross from the surface. The product is an opaque yellow crusty substance, easily broken up and crushed into a fine powder which will pass a 100's mesh. When litharge is fired it produces a smooth clear yellow glaze on light-firing clay, usually with no pitting, and a brown to olive-green glaze on iron-bearing clays under reduction. Although litharge will produce a glaze around 860C, to bring the glaze to maturity it should be fired in the range 950-1100C or more.
lead carbonate (2PbCO3 +Pb(OH)2)
Lead carbonate forms as a white crust on lead plates subjected to the prolonged action of carbon dioxide and acetic acid vapour. The material's fine and flaky particles allow it to remain suspended in water or slip, and it is the most suitable form of lead for application by pouring or dipping. A glaze may be formed in the mid-to-high 800's, but around 950-1100C lead carbonate is at its best with a shiny uniform surface.
Glaze Application
binders
Liquid glaze applied in any quantity to dry clay will almost certainly result in cracking of the clay body, consequently for a normal glaze application the pot must be damp enough to inhibit the absorption of glaze water.
When the body of a damp pot shrinks in drying it is likely to contract at a different rate from an applied glaze or slip and cause it to flake off the surface. To avoid flaking the usual strategy is to mix the glaze ingredients with a binder which will contract evenly with the damp pot and retain its adhesiveness. A binder or siccative will prevent the glaze from becoming detached from the surface of the pottery during drying and kiln loading. It is difficult to retain lead glaze on a damp unfired pot without using a binder of some kind.
Organic Binders
Starch, gelatine and gum arabic are adequate organic binders. Flour is especially effective and reliable on raw clay vessels. When mixed with water to a creamy consistency, with or without glazing ingredients, it remains fixed to the surface during drying and shrinking.When dry it forms a hard coating which allows pots to be handles during transport and kiln loading. An organic binder does not become a constituent of the glaze. During firing it will burn out completely and, in the case of flour, appears to leave no archaeologically detectable trace.
Non-organic binders
The most common non-organic binder is clay from which the pot is made, or a different clay with a compatible rate of shrinkage. Applied to the damp surface in the form of a slip containing glaze ingredients, it contracts uniformly with the pot during drying with no risk of flaking. When dry, it can be handled without any glaze loss.
Although ideal in this respect, the clay becomes a constituent of the glaze and will greatly influence glaze behaviour. Many clay binders will make a glaze less fusible and create an underfired appearance unless fired to a sufficiently high temperature.
The clay to be used for the binder may be sieved or levigated to remove particles of sand and grit. The presence of excessive sand or grit in a medieval glaze is a fairly safe indicator of the use of unseived clay as a binder. A slip binder prepared from body clay by fine sieving or levigation will sometimes fire to a lighter colour than the body of the pot. In such cases a faint underglaze slip, roughly coinciding with the glazed area, is likely to be the remains of a clay binder applies either for a sprinkling application or for brushing and pouring.
A clay binder may also serve to introduce additional silica, which readily combines with the lead and can increase the gloss and fluidity of the glaze when an adequate temperature is maintained. Iron and other oxides in the clay can influence glaze quality and colour.
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