Thursday, September 24, 2009

Technical considerations

Porcelain--note platey structure. Figure 1a (should be below)

Updraft kiln, firebox separate Figure 7c

Chinese Dragon Kiln Figure 7b

Bonfire/Neolithic kilns Figure 7a

Tin glaze Figure 6

German Salt Glaze Figure 5

Roman sigillata body and slip Figure 4

Faience Figure 3

Vegetable temper (above) and Roman smooth body (below)
Figure 2

Calcareous clay (above)/Limey earthenware Figure 1b/c (note: porcelain image at top)

Hello Class, Here are the technical notes I showed you in class. Remember, they are very basic!

Technical Considerations
• Clay: A fine-grained natural material which, when wet, is characterized by its plasticity, the property which allows it to be deformed by pressure into a desired shape without cracking and to keep this shape when the pressure is removed. In addition to clay minerals, clay typically contains feldspar, calcite and iron oxide.
• Clay Minerals: A group of very fine-grained minerals (alumino silicates) which are the main constituents of clay. They occur as minute platelets which, when wet, slide across one another, giving the clay its plastic properties.

Clay Structure (Ceramic Masterpieces) fig1.8 p38 particles of Kaolinite. Fine platelets occur as stacks that require intensive mixing to break up into individual particles. When separated by water films, the platelets slide over one another to give good plasticity to a clay-water paste. (above figure 1a)

figure 13.2 p. 235 Calcereous illite clay from Corinth consists of much finer platey particles and has high impurity content (potassia, calcia, iron, titania); Limey earthenware clay from Iran more impure, finer particle, less-well developed platey nature. (figures 1b/c)

Primary and Secondary Clays
Clay occurs naturally in beds of weathered and decomposed granite and gneiss that makes up 85% of the earth’s surface.
Primary clays are found on the site of the parent feldspathic rock and are relatively pure, containing only materials that were part of the parent rock such as feldspar, quartz and mica. They are typically white, large grained and aplastic with low shrinkage. They have a high melting point and require flux to decrease firing temperature (i.e. kaolin).
Secondary clays are deposited away from the parent rock by water or wind and are characterized by fine, uniform particles. They tend to be very plastic and require the addition of other materials such as sand or grog to prevent excessive shrinkage.
Calcareous Clay: contains more than 5% lime, usually in the form of calcite (calcium carbonate, CaCO), the main constituent of limestone. Clays of this type are typically alluvial and were widely used in the Mediterranean and the Near East. Calcareous clays fire to a very stable structure in the range of 850-1050 degrees C, characteristic of much ancient pottery. Careful control of temperature was not necessary and a wide range of colours would result from a single firing.

• Potters clean and refine their dug clay by levigation--washing clay in slurry form, allowing it to settle and pouring off liquid with finer particles in suspension into additional tubs. It is generally aged and foreign particles are removed. Often sand, crushed pots, shell or other materials are added to improve the clay body, to give it bulk, strength, ability to withstand rapid changes in heat etc. Washed clay needs to be wedged (pugged) to eliminate air bubbles and foreign materials. In factory situations, specialized workers look after each of these tasks.
Temper in Pots
Micrographs showing vegetable temper (top) and numerous grains of very fine sand or silt in romanised ware (bottom). Note the regular, well-sorted size distribution of particles. The inclusion of temper improves thermal shock resistance in bonfire firing and cooking. Hand-pots break less-easily than do wheel-thrown wares. (PMCT p.58) (Figure 2 above)

Forming techniques:
Methods are direct (throwing, hand forming, carving, turning, modelling) or indirect (moulding). Clay can be squeezed, pinched and paddled into shape. Pinch or thumb pots most familiar--limit to size, but otherwise, any level of sophistication possible. Pinching and hand forming can also be done to embellish a thrown pot--ie, additional ornaments, lip or foot design. Hand-building can involve cutting slabs of clay to patterns and luting edges together. Coil building can simulate wheel-throwing in roundness, uniformity. Often walls thinned and raised by beating with paddle, supporting wall from inside of pot with “anvil.” Huge pots made in this way still in Greece, Asia. Finished pots may be burnished with stone, shell or mineral like hematite to align particles/consolidate surface and gives degree of impermeability and beautiful sheen.
Molding technique originated with bronze casting, carried over to ceramics. Single valve (i.e. sprig molds) designs fairly flat but can later be undercut. Efficient way to produce identical or nearly-identical multiples--sometimes hand finishing makes each one subtly different. Romans industrialize ceramics through molds. Double-valve molds--two sides to mold fit together, side seams smoothed over--enable replication of eccentric shapes that otherwise would require very slow building techniques. Slip casting more efficient--wet slip poured into mold, sloshed around, excess poured off (can be repeated)--shrinkage makes clay pull away. Cutting: can elaborate surface by piercing, cutting patterns into it. Might also involve impressing various tools, seals; inlaid patterns can later be filled in with contrasting coloured slip (Korean celadons from 10-14th c.) or as intensification of glaze (Sung-Dynasty).

Potter’s Wheel
consists of top circular bat, bearing or bearings, on which wheel rotates, heavy flywheel that can be turned by assistant or by foot and which will turn for a long time, leaving hands free to work. Very tall pots usually thrown in sections and joined--wet clay will only support so much weight for thickness of walls. Accurate throwing might involve templates, pointers for measuring. Clay can be thrown into concave molds fixed to bat--useful for mass-production of identical items, especially bowls with relief-molded decoration. Jiggering--mold fixed to bat, template made for external radius--mold is usually convex--place pancake or bat of clay over mold, lower arm over spinning clay--press until excess is shaved away. Turning: shaving-down of green-hard pot to thin walls--refines shape--Chinese eggshell porcelains (18th c) shaved to almost unbelievable thinness. Used to perfect profiles of shoulders, rims, feet, flanges etc.

Slips and Glazes:
make pots impervious (for earthenware) or stronger, more chip-resistant and enhance appearance. Slips are clay particles suspended in water, often have materials added to enhance suspension and/or colouring agents. Slips have natural affinity for clay body. Glazes have additional fluxes added and consist mainly of colouring agent--metal or metalic oxide, with glassy agent--silica (quartz, flint or sand) plus flux (lead, soda, wood-ash, borax or magnesia) to lower melt temperature of silica plus frit, clay, feldspar etc. to give “body” to glaze. Raw glazes--naturally insoluble with limestone and feldspar as main fluxes or lead glazes and fritted glazes in which soluble alkalis are rendered insoluble by fritting--melting with silica and later milled and sifted for uniform particle size.
Fritting renders toxic ingredients like heavy metals, lead non-toxic. Soluble alkalis are problematic because the effloresce on the surface during drying and result in non-uniform composition with poor surface qualities.
Glaze materials fire different colours under different conditions. Main difference created by oxidizing--clear, bright flame, lots of oxygen--colouring agents remain in oxide form versus reduction--smoky, choked flame, “robs” oxygen from glaze or body of pot (oxygen combines with excess carbon in atmosphere), creates different colours--coloring agents are the metals themselves, not in oxide form. Alternations in environment can cause interesting speckles, different pots in same batch to fire differently, and are essential to Greek red and black pots.
Salt-glaze and Wood Ash--unique--enhance characteristic of clay itself--used on stoneware. Salt (NaCl) thrown into kiln over 1100 degrees C. decomposes, releasing chlorine gas. Silica acts as powerful flux on surface of pots--fuses ahead of rest of body, causing shiny glassy surface, often speckled. Very popular in Germany late middle-ages. Wood ashes dusted on pots prior to being put into kiln or exposed to ash from fire in kiln--effect is rustic, “natural”, highly-sought after in Japan.
Transfer printing important industrial technique--first used in Liverpool or Worchester for soft-paste porcelain in 1756. Makes use of copper plate engraving technology --copper plate is engraved--pigment applied to roller, plate wiped clean (pigment remains in grooves)--plate is printed onto thin paper attached to wares. Ware is then glazed and fired. Alternate process uses gelatin bat to deposit oil onto glazed wares, which are dusted with overglaze pigments and low-fired.

Faience (Figure 3)
Faience: Analysis of Egyptian faience reveals there is no deliberately added clay. The body was composed of crushed quartz, as shown in the micrograph, with small amounts of glass that bound the quartz grains together. A continuous glassy layer covers the surface. The body is porous, with holes showing as black. The glaze improves appearance and stabilizes surface/faience object. (PMCT p.104.)
Roman Terra Sigillata Bodies and Slips (figure 4)
Roman factories were able to impose standardization through the use of fine, calcareous clays that fired to a consistent quality over a range of temperatures ranging from 850-1050 degrees common to wood fired updraft kiln. Glossy surface is achieved by use of a very fine slip oxidized to sealing-wax red. Micrograph of sherd of Eastern Gaulish sigillata shows open, partially-vitrified body and vitreous slip. The two adhere well with the resulting tough “non-stick” surface that made this ware popular for the table. (PMCT p.191)

Salt-Glazing (figure 5)
The method of salt-glazing differs from other forms of glaze in that instead of raw glaze being applied to a pot, the glazing compound—salt—is deposited in the kiln during firing. Salt (NaCl) decomposes above 1100 degrees C to produce sodium oxide plus hydrogen chloride gas. The sodium oxide reacts with (fluxes) the alumina and silica of the pot to form a thin glaze layer. The glaze is rough to the touch due to numerous small flaws. Micrograph shows less than 0.1mm thick glaze on 16th c. stoneware from Frechen. (PMCT 124)

Tin-Opacified Glazes (figure 6)
Tin-glazes were first developed by Islamic potters in Iraq in 8th c. CE. Tin created appearance of opaque white glaze, which was excellent for decoration. Tin and lead were fired together to produce oxides; this was added to an alkali-silicate mixture. Micrograph shows section through body and glaze—numerous fine white particles are tin. (PMCT p.111)

Kilns: Simplest is bonfire--pots are stacked on wood, additional fuel packed in around--set on fire. Reduction atmosphere created by packing in dung or organic material--many primitive and ancient pots fired this way, leaving very few traces for archaeologists. Evolution to separate fire chamber from pot chamber--often with simple ceramic or metal grate. In bank kilns--China--single fire chamber at bottom of hill, succession of pot chambers go up hill--as get further from fire chamber, successively cooler (permits multiple firings at once). Dragon kilns 12th c. Song Dynasty two meters across and 30 meters long, rise up hill at 15 degrees, thick insulation, entry ports along way for insertion of fuel--fire moved up the hill during process. Air pre-heated by passing over wares below fire; exhaust gasses passed over wares ahead of flame, pre-warming wares, making efficient use of heat.

Kilns (from Ceramic Masterpieces, David Kingery and Pamela Vandiver)
Left, above: simple bonfire kiln
Left, below, Neolithic kiln, China (Figure 7a)

Chinese Dragon Kiln, 12th c. (figure 7b)

Updraft Kiln, firebox separate from wares (Kingery and Vandiver) (figure 7c)

Updraft kilns--heat is drawn up from fire pit, across pots and up chimney. Down-draft kilns--heat is drawn up and across, and then down again to flues at bottom of chamber--more efficient use of fuel, necessary for stoneware and porcelain temperatures. Saggars and muffles used to protect pots from flames. Modern kiln technology completely different--electric, controlled, wares stacked on trolleys that travel through kiln to produced identical wares. Control of temperature essential. Renaissance kiln masters familiar with relationship of colour to heat: red--525 C; Bright Red, beginning orange 1000C; Dull white 1300C; Bright white 1400C. Would open flues to allow hot gasses to reach cooler parts of furnace, stoke etc. to control heat. Firebox design and number important to stoneware or porcelain heat.

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