Chemistry:Qom technique

Short description: Cementation-glazing technique for siliceous faience associated with Qom, Iran

Turquoise-glazed kharmohreh produced using the Qom technique

The Qom technique is a term used in archaeological and archaeometric literature for a form of cementation glazing used in the production of faience, a non-clay siliceous ceramic with a vitreous surface. The technique takes its name from Qom, Iran, where Hans E. Wulff, Hildegard S. Wulff and Leo Koch documented the process in use in the 1960s for the manufacture of turquoise-blue beads and related objects. In Iran such objects are generally known as kharmohreh (Persian: خرمهره).

The term Qom technique refers specifically to the glazing process rather than to a distinct ceramic material or style. Its use records the modern Iranian context in which cementation glazing was first documented ethnographically by researchers; it does not in itself indicate that the technique originated in Qom.

Material and terminology

Faience of the type associated with the Qom technique differs fundamentally from ordinary glazed earthenware. Its body consists predominantly of finely divided quartz, quartz-rich sand or another silica-rich material rather than clay. During firing, a relatively thin alkaline-lime-silica glass layer forms at the surface. Copper compounds commonly provide the characteristic turquoise or blue-green colour.

Studies of ancient faience production generally distinguish three principal glazing methods: application glazing, efflorescence glazing and cementation glazing.

In application glazing, a glaze slurry is applied directly to the surface of the formed object, for example by brushing or dipping. In efflorescence glazing, soluble alkali salts are incorporated into the body; as the object dries, these salts migrate towards the surface and form a layer that vitrifies during firing. Cementation glazing differs from both processes because the formed and dried object is buried completely in a dry glazing powder before firing.

Both efflorescence and cementation are therefore often described as forms of self-glazing, although the physical and chemical mechanisms by which the surface glaze is produced are different.

Process

In the Qom or cementation technique, the silica-rich body is first shaped and dried. The object is then buried in a powdered glazing mixture, traditionally placed within a refractory container. The mixture documented and discussed by Wulff and later researchers included alkali-bearing material, copper compounds, calcium oxide or calcium hydroxide and silica. Charcoal was also reported in the Qom process documented by Wulff.

The container is fired to a temperature of approximately 1,000 °C (1,830 °F). During firing, alkalis and copper-bearing compounds from the surrounding powder interact with the silica-rich surface of the object and a glassy glaze develops. The copper gives the resulting glaze its characteristic blue or turquoise colour.

The mechanism is more complex than a simple transfer of molten glaze from the surrounding powder to the object. Experimental work by Mehran Matin and Moujan Matin identified more than one process contributing to glaze formation. Alkalis can migrate from the surrounding powder through direct interaction at the interface between the powder and the siliceous body, while volatile alkali and copper compounds can also contribute to reactions at the surface.

An important feature of successful cementation glazing is the behaviour of the surrounding powder during firing. It must become sufficiently coherent to create appropriate firing conditions while remaining porous and friable enough to be removed afterwards. It must also avoid adhering permanently to the newly formed glaze. Matin and Matin described the formation of a partly sintered capsule around the object as an important element of the process.

After firing and cooling, the surrounding material is broken or crumbled away and the glazed object is removed. The process is particularly efficient for producing many small objects, such as beads, because numerous pieces can be buried and glazed during the same firing without each object requiring an individually applied glaze.

Role of the glazing ingredients

The silica-rich body provides the principal structural material of the object. Alkalis act as fluxes, reducing the temperature at which silica can participate in the formation of glass, while lime contributes to the stability and composition of the glaze. Copper compounds act primarily as colourants, producing blue and turquoise hues under suitable oxidising firing conditions.

The exact compositions used historically need not have been uniform. Plant ash is one possible source of alkalis, and different ashes can introduce varying quantities of sodium, potassium, calcium, magnesium, chlorides and other compounds. This makes it difficult to reconstruct a single universal recipe for cementation glazing from the composition of surviving objects alone.

The role of charcoal in the process has also been reassessed. Although charcoal was reported as an ingredient in the Qom glazing mixture recorded by Wulff, replication experiments by Matin and Matin showed that successful cementation glazing could be achieved without it. They suggested that charcoal was therefore not necessarily an essential technological component of the process.

A subsequent experimental study by the same authors investigated cattle dung ash as a possible alkali source. Objects glazed by cementation using dung-derived ash were successfully fired at about 980 °C (1,800 °F), demonstrating that several readily available natural ash sources could potentially provide the fluxes required for the process.

Research history

Cementation glazing became an important subject in the study of ancient vitreous materials after Wulff, Wulff and Koch published their observations of bead production in Qom in 1968. They compared the Iranian process with ancient Egyptian faience and proposed that the technique observed in Iran might preserve technological principles of much older siliceous-glazing traditions.

At approximately the same period, experimental research into ancient faience demonstrated that more than one method could produce the characteristic glazed quartz body. Joseph V. Noble's experiments emphasised an efflorescence or self-glazing process in which soluble salts incorporated into the body migrated to the surface during drying.

Later studies by Michael Tite, Ian Freestone, Margaret Bimson and other researchers established the modern analytical framework in which application, efflorescence and cementation are treated as distinct but sometimes difficult-to-distinguish glazing technologies.

Laboratory replication experiments showed that the different techniques tend to produce different relationships between the glaze, the intermediate or interaction layer and the quartz-rich core. Cementation-glazed objects commonly exhibit relatively thin glaze and interaction layers and comparatively little interparticle glass within the body. Efflorescence can produce more extensive glass between quartz particles because the flux is already distributed through the body before firing.

These criteria are useful but not absolute. Analytical research on ancient Egyptian material has shown that manufacturing conditions, raw-material composition, firing temperature and the quantity of flux can alter the microstructure considerably. Consequently, it is not always possible to assign an archaeological faience object to one glazing method with certainty.

Kharmohreh and Qom

The objects that brought the Qom technique to the attention of researchers were small turquoise-blue objects traditionally manufactured in Qom, particularly beads commonly referred to as kharmohreh. Their appearance resembles a much wider family of ancient blue and turquoise siliceous materials that were produced in Egypt and parts of the Near East over several millennia.

The technological importance of the Qom workshops lies not in proving Qom as the place of origin of ancient faience, but in providing researchers with a documented example of cementation glazing as a living or recently surviving craft practice. Observation of the Iranian process supplied an experimental model against which the microstructures of archaeological faience could be compared.

For this reason, "Qom technique" became established in some archaeological literature as a convenient synonym for cementation glazing. Later archaeometric studies continue to employ the term while treating the process within the broader technological history of faience production rather than as an exclusively Iranian invention.

Archaeological significance

The documentation of the Qom technique had significance beyond the study of Iranian craft traditions. It demonstrated experimentally that a quartz-rich object could acquire a continuous glaze without the glaze being painted or dipped onto its surface and without soluble glazing salts having to be incorporated throughout the body.

The technique therefore provided archaeologists with another possible explanation for the manufacture of ancient faience objects whose microstructure did not correspond closely to application or efflorescence glazing. Subsequent replication and microscopic studies confirmed that cementation can produce structures comparable to those found in some archaeological faience.

At the same time, modern research has moved away from treating any one glazing technique as a universal explanation for ancient faience manufacture. Different methods, combinations of methods and variations in raw materials were used at different times and places. The Qom technique is consequently best understood as one technological solution within the much broader history of silica-based glazed materials.

See also

• Egyptian faience
• Glaze
• Fritware
• Stonepaste
• Iranian pottery
• Ancient Egyptian technology