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silica (SiO2) and water (H2O) differ in various types of clay (Eiduks 1935–1936, 25494). Clay found in
nature may have different admixtures – quartz, feldspar, mica, calcite dolomite, iron oxides etc. The less
admixtures, the purer is clay and more valuable for economic use. Pure clay is white. Light, very qualitative
clay is found in Rheinland, especially in the vicinity of Cologne. Oxides tint clay differently. The colour
of the red clay that is widely available in Latvia derives from admixture of iron oxides.
Clays that are distributed within Latvia’s territory have formed during various geological periods.
Devonian clay deposits are distributed in the northern and north-eastern part of Latvia as well as in the
northern part of Kurzeme. Gauja suite clay is exposed on the banks of deep gullies and rivers, especially
between Sigulda and Valmiera. Quaternary clay deposits are found in Zemgale, Venta–Usma river basin,
and in the north-eastern part of Latgale region, in the vicinity of Lake Lubāns. Triassic Nemunas suite
clay is encountered only in the vicinity of Saldus town and Liepāja city (Segliņš, Brangulis 1996, 11).
All these clays are well suitable for making building ceramics.
Classification of ceramics
Ceramics imply baked clay articles. Usually baking is done at the temperature of 900–1000 °C, in
oxidising atmosphere, which means that during the process of baking air is introduced into the oven.
When baked at lower temperature, ceramics may be fragile. During baking at 900 °C, iron oxide is
formed in red clay, which tints the ready-made article orange-red. In case during the process of baking
no additional oxygen is introduced, the atmosphere in the oven is reductive, and ceramic articles become
dark and sooty – thus red clay brick becomes black. If the baking temperature in the oven exceeds
1000 °C, red clay becomes darker and darker red, to carmine, after that it is tinted brown but at 1200 °C
it becomes grey (Davey 1961, 65). However, some types of clay cannot be baked at such high temperature
because then they start melting and become fragile.
The versatile use of ceramics in the Middle Ages was determined by the positive properties of the raw
material itself. Wet clay is plastic and it is easy to be shaped, it can easily be decorated with a relief décor
that is impressed into the surface. Ceramic articles that have been baked at high temperature are almost
as hard as stone, they are waterproof and little subjected to the effect of heat and humidity. Almost the
only negative property of ceramics is its lack of impact strength.
Since ceramics is fragile, only a small part of medieval products have preserved their initial appearance
until the present day. Damaged articles and their small fragments have been thrown away already during
the time of construction, items having served their time have been thrown away as garbage during
repair and reconstruction works, and after destruction of buildings during attacks or abandonment of
houses during peacetime the building materials of walls and roofs make the debris forming a cultural
layer. Building ceramics is an inorganic material and articles are preserved underground almost in any
soil layers, without changing their appearance for centuries. Therefore, in archaeological excavations,
ceramic fragments make a category of mass finds that can reach hundreds and thousands even within a
rather small excavation site.
In order to scientifically handle, study and date such multi-numbered material, it is necessary to divide
it into smaller groups indicating the common and distinctive properties (Bauer 1993, 37). Depending on
Ieva Ose. Building ceramics of Turaida Castle in the 13th–17th centuries