UPMSP Vocational Ceramics cheat sheet

Silica (SiO2) is the refractory skeleton former; fluxes (Na2O, K2O, CaO, MgO, PbO, B2O3) lower melting point; alumina (Al2O3) raises maturing temperature and gives body strength and glaze durability.

Three main clay families: kaolin (kaolinite, Al2O3·2SiO2·2H2O, low plasticity, high refractoriness, ~1780 °C melting); ball clay (fine-grained, high plasticity, high shrinkage, organic content, darker fired colour); fireclay (refractory, moderate plasticity, used in stoneware and kiln furniture).

Typical earthenware body: 25-40 % ball clay, 25-35 % kaolin, 20-30 % silica, 5-15 % flux (feldspar or talc). Stoneware: 25-30 % ball clay, 25-30 % kaolin, 25-30 % silica, 15-20 % feldspar, fired 1200-1300 °C. Porcelain: 50 % kaolin, 25 % silica, 25 % feldspar, fired 1250-1400 °C.

Non-plastic additions: silica (reduces shrinkage, controls thermal expansion), feldspar (flux, vitrification), talc (flux in low-fire bodies, reduces expansion), grog or chamotte (fired clay, reduces shrinkage and drying cracking, improves thermal shock resistance), whiting/limestone (low-fire flux, causes lime blowing if coarse particles above ~1 mm).

Plasticity depends on particle size (finer = more plastic), water film between platelets, and clay mineral type; bentonite (montmorillonite) is 3-5 times more plastic than kaolin and is added at 1-3 % to boost plastic bodies.

Shrinkage: drying shrinkage 5-8 % typical, firing shrinkage 3-8 %, total 8-15 %. Measure by marking a 100 mm wet bar and measuring after drying and after firing; shrinkage % = (wet length - fired length) / wet length × 100.

Water of plasticity is the water needed to make clay workable, typically 18-25 % of dry weight; higher for ball-clay-rich bodies, lower for grogged bodies.

Clay minerals: kaolinite (1:1 layer, low cation exchange), illite (2:1, non-expanding), montmorillonite (2:1, expanding, high cation exchange capacity, causes high shrinkage and drying cracks).

Refractoriness test: pyrometric cone equivalent (PCE) of a clay; kaolin PCE ~34 (about 1750 °C Orton), fireclay ~31-33, ball clay ~26-30.

Blunging: mix clay and water in a blunger to a slip of 1.4-1.7 g/cm3 density; add 0.2-0.5 % sodium silicate or sodium carbonate as deflocculant to reduce water and viscosity.

Sieve and magnet: pass slip through 60-100 mesh sieves and a magnetic separator to remove iron particles (cause of black specks and iron spots after firing).

Filter press: removes excess water from slip to produce filter cake of 20-25 % moisture, then pugged or wedged.

Pug mill (de-airing): vacuum of 600-700 mmHg removes air, produces dense, homogeneous, workable clay; extruded slugs are more plastic and shrink less than non-de-aired clay.

Ageing (souring): storing damp clay for weeks allows bacteria and organic matter to develop, improving plasticity and workability, especially for ball-clay-rich bodies.

Dry mixing: weigh each ingredient to ±1 %, mix dry powders for 5-10 minutes before adding water to avoid uneven distribution of fluxes (causes warping and colour variation).

Water content check: a good throwing body is 18-22 % water; hand-rolled coil should crack only slightly when bent around a 25 mm mandrel.

Slaking test: dry clay lump in water should slake within minutes for ball clay; kaolin slakes slowly. Slaking indicates plasticity and ease of blunging.

Wedging purposes: removes air pockets, homogenises moisture and body, aligns particles; spiral wedging for small batches, cut-and-stack (wire wedging) for large batches.

Rule: wedge clay in a consistent direction; trapped air pockets cause S-cracks in bases and blowouts in firing.

Hand-building methods: pinch, coil, slab, and press-mould. Coils should be joined with slip and scored (cross-hatched) joints for strength.

Slab building: roll slabs evenly to 6-10 mm; place between cloth or paper to prevent sticking; allow leather-hard state for joining to avoid slumping.

Wheel throwing: centre clay at 300-400 rpm for a 1 kg ball; open with thumbs; pull walls in 2-3 pulls; final wall thickness 4-8 mm for functional ware.

Throwing terms: centring, opening, pulling, shaping, trimming (turning) at leather-hard, attaching handles at soft leather-hard.

Jiggering and jolleying: industrial forming of flatware (jigger) and holloware (jolley) on a rotating plaster or metal mould.

Extrusion: pug mill or extruder produces constant cross-section shapes; moisture must be uniform to prevent twisting and cracking.

RAM pressing: pressure casting of flatware in porous moulds under 1-2 MPa pressure, faster than slip casting.

Drying stages: wet (plastic) → leather-hard (can be trimmed and joined) → bone dry (no free water, ~0-2 % moisture) → fired. Never fire ware above 3-5 % moisture.

Drying shrinkage happens mainly during water loss from 25 % down to 8 %; drying too fast causes uneven shrinkage and cracks. Dry slowly in humid conditions, cover with plastic for the first 24 hours.

Bisque firing: 950-1050 °C, slow heating (50-100 °C/h) up to 600 °C to burn out carbon and chemically combined water; soak at top temperature 20-30 minutes.

Glaze firing: earthenware 1000-1150 °C, stoneware 1200-1300 °C, porcelain 1250-1400 °C; heating rate 100-150 °C/h, cooling controlled to avoid dunting (thermal shock cracking).

Quartz inversion: β-quartz to α-quartz at 573 °C on heating and cooling, accompanied by ~0.8 % volume change; slow the kiln through 500-600 °C both ways.

Cristobalite inversion at 200-270 °C on cooling can cause dunting in high-silica bodies; cool slowly through 300 °C.

Oxidation vs reduction: oxidation with excess air gives clean colours; reduction (starved air, CO present) gives celadons, copper reds, iron speckling. Control by damper and air intake.

Pyrometric cones: Orton cones deform at specified temperatures at a given heating rate (e.g., cone 06 = 999 °C, cone 04 = 1060 °C, cone 6 = 1240 °C, cone 10 = 1300 °C at 60 °C/h). Place cones in a cone pack at kiln top, middle, bottom.

Kiln types: electric (oxidation, clean, controlled), gas (reduction possible), wood (ash deposit, flashing), raku (fast firing and reduction in combustible material).

Firing faults: bloating (overfiring or excess organic matter), black coring (carbon trapped by fast early firing), warping (overfiring or uneven body), dunting (cooling too fast through quartz inversion).

Glaze formula: silica (glass former) + flux (melter) + alumina (stabiliser, prevents running) + colourants/opacifiers. Typical glaze: 40-60 % silica, 10-25 % flux, 10-20 % alumina.

Glaze fit: glaze thermal expansion must be slightly lower than body so glaze is in compression (prevents crazing). Increase silica or reduce flux to lower expansion; add frit or boron to increase expansion.

Crazing (fine crackle) = glaze in tension (expansion too high); shivering (flaking) = glaze in compression (expansion too low).

Slip: clay + water + deflocculant, density 1.4-1.7 g/cm3; casting slip 1.7-1.8 g/cm3, deflocculated with sodium silicate; decorating slip 1.3-1.5 g/cm3.

Surface decoration: slip trailing, sgraffito (scratching through slip to reveal body), mishima (inlay into incised lines), wax resist, cuerda seca, maiolica/tin-glaze painting, underglaze and overglaze decoration, decal transfer.

Oxide colourants: iron oxide (Fe2O3) 1-10 % gives amber/celadon in reduction, tenmoku at 8-10 %; copper oxide 0.5-3 % gives green in oxidation, red in reduction; cobalt oxide 0.5-2 % gives blue; manganese gives purple-brown; chrome gives green (with cobalt) or pink (with tin).

Opacifiers: tin oxide 5-10 % (white, expensive), zirconium silicate 8-12 % (cheaper white opacifier), titanium dioxide (varies, can crystallise).

Glaze application thickness: 0.3-0.5 mm dry, achieved by dipping 1-3 seconds in a well-stirred glaze of density 1.3-1.5 g/cm3; use glaze thickness gauge to check.

Firing faults in glaze: pinholing (bubbles, often from dust or thick application), crawling (glaze pulls away from body, from dusty or oily surface), running (too much flux or thick application), shivering, crazing.

Centring is the foundation: apply even pressure with both hands, elbows braced against thighs, wheel at moderate speed; off-centre clay cannot be thrown thin without distortion.

Opening: press thumbs down into centre, leaving 8-10 mm base thickness; measure base with a needle tool.

Pulling: start at base, pull upward in 2-3 passes, keeping walls even; compress the rim to prevent S-cracks and rim cracks.

Trimming (turning): at leather-hard, place pot upside down on wheel, centre, and turn foot ring with a loop tool; foot ring gives stability and a clean finish.

Hand-building: pinch pots start from a ball, press thumb into centre and pinch walls evenly to 6-8 mm; coil pots join coils with slip and score, blend with a rib.

Slab work: roll slabs evenly with a rolling pin or slab roller; use battens (thickness guides) for uniform thickness; join slabs at leather-hard with slip and score.

Ribs, wooden modelling tools, loop tools, wire cutters, calipers, and throwing sticks are the core tool set; keep tools clean and dry to prevent rust and contamination.

Drying ware: cover with plastic sheeting and dry slowly; place pots upside down to even drying; avoid draughts that cause one-sided drying and cracks.

Plaster of Paris (calcium sulphate hemihydrate, CaSO4·½H2O): mix plaster to water at 100:70 by weight (plaster:water) for moulds; 100:80 for slip-casting moulds (more porous).

Mixing: sprinkle plaster into water, soak 1-2 minutes, stir gently for 1-2 minutes to avoid air bubbles, then pour; setting time 15-25 minutes; do not remix set plaster.

Plaster sets by hydration to gypsum (CaSO4·2H2O) with slight expansion (~0.3 %), which reproduces detail accurately.

Mould types: one-piece (open) moulds for simple shapes; two-piece and multi-piece moulds for complex shapes with undercuts; case moulds with plaster or rubber master.

Release agents: soft soap solution (green soap) for plaster-to-plaster; shellac or parting compound for plaster-to-clay; apply several coats and let dry.

Slip casting: pour deflocculated slip (density 1.7-1.8 g/cm3) into dry mould; mould absorbs water, builds wall thickness (e.g., 3-5 mm in 10-30 minutes); pour out excess (hollow cast) or leave to set (solid cast).

Casting faults: uneven wall (mould too wet or slip too thick), pinholes (air bubbles in slip), warping (demoulding too early), cracking (drying too fast).

Mould care: dry moulds thoroughly between casts (24-48 hours in a drying room at 30-40 °C); never heat plaster above 60 °C or it dehydrates and loses strength; store dry and dust-free.

Plaster mould life: typically 50-150 casts depending on care; surface degradation (chalking, loss of detail) signals replacement.

Design process: brief → research → sketches → scale drawings → models/maquettes → prototypes → testing → production drawings → batch production.

Technical drawing conventions: orthographic projection (front, side, top), isometric views, dimension lines in mm, section views showing wall thickness.

Design constraints: shrinkage allowance 8-15 % must be added to mould dimensions; minimum wall thickness 4-6 mm for strength; avoid sharp internal corners (stress concentration).

Ergonomics: mug handle opening 25-30 mm for fingers; plate rim width 25-40 mm; bowl depth-to-diameter ratio 0.4-0.6 for stability.

Product development: cost, market, function, aesthetics, safety (food-safe glaze, lead-free), and manufacturability (mould complexity, casting time, firing yield).

Sketching tools: graphite pencils (HB, 2B), technical pens, drawing board, T-square, set square, compass, scale ruler; keep a design notebook with dated entries.

Prototype testing: water absorption (boil test), thermal shock (oven to ice water cycles), dishwasher resistance, chip resistance, food safety (lead and cadmium leaching test).

Colour and surface design: develop a colour palette, test tiles for glaze combinations, document glaze recipes and firing cycles for reproducibility.

Silica dust is the main hazard: crystalline silica causes silicosis. Use wet methods, local exhaust ventilation, N95 or better respirators, and never dry-sweep clay dust - wet-mop or vacuum with HEPA filter.

Electrical safety: kilns draw high current (e.g., 30-50 A at 230 V for a large kiln); use correct cable gauge, RCD/GFCI protection, and keep water away from kiln and controls.

Kiln safety: never open a kiln above 200 °C; wear heat-resistant gloves and face shield; keep combustible materials 1 m from kiln; ensure ventilation for fumes (CO, sulphur, lead).

Plaster safety: wear dust mask when mixing; plaster dust is an irritant; never pour set plaster down drains (it sets and blocks pipes).

Tool maintenance: sharpen loop and trimming tools, replace worn ribs and sponges, oil wheel-head and bearings per manufacturer schedule, check kiln elements for sagging or breakage and replace as a set.

Wheel maintenance: keep wheel head dry and clean, check belt tension, lubricate bearings, and ensure foot pedal and splash pan are secure.

First aid: burns - cool with running water 10-20 minutes; cuts - clean and cover; eye splash - flush 15 minutes; report all incidents and keep a first-aid kit stocked.

Personal protective equipment: apron, safety glasses, dust mask, gloves for glaze chemicals; label all containers with contents and hazard symbols.

Costing formula: total cost = direct materials + direct labour + firing/energy + overheads (rent, depreciation, admin) + packaging + wastage allowance (typically 5-15 %).

Selling price = total unit cost / (1 - desired profit margin) or total cost × (1 + markup %). Example: cost ₹200, 30 % margin → price = 200 / 0.70 = ₹286.

Break-even quantity = fixed costs / (selling price - variable cost per unit); break-even point in sales value = fixed costs / contribution margin ratio.

Pricing methods: cost-plus, market-based (competitor pricing), value-based (perceived quality), and penetration pricing for new products.

Marketing mix (4 Ps): product (design, quality, packaging), price, place (direct sales, craft fairs, online marketplaces, export), promotion (social media, catalogues, trade fairs, workshops).

Record keeping: maintain a costing sheet per product, stock register for raw materials, sales and expense ledger; track yield (percentage of fired pieces that pass quality check).

Legal and compliance: GST/ VAT registration where applicable, food-contact safety certificates for tableware, export documentation (invoice, packing list, certificate of origin), trademark registration for brand.

Scaling up: batch production, jiggering/RAM pressing for volume, standardising glaze recipes, training workers, and quality control checkpoints at each stage.

Silicosis risk from respirable crystalline silica (<10 µm particles); exposure limits are typically 0.05-0.1 mg/m3 as respirable dust over an 8-hour shift; monitor with air sampling.

Lead and cadmium in glazes are toxic: use lead-free glazes for food-contact ware; handle lead compounds with gloves and respirator; test fired ware for leaching (e.g., ASTM C738 or ISO 6486 limits).

Ventilation: local exhaust at mixing, sieving, and spraying stations; general dilution ventilation for kiln rooms; kilns must vent combustion gases outdoors.

Waste hierarchy: reduce, reuse, recycle, recover, dispose. Reuse clay scraps (slake and re-wedge), reclaim glaze drips, recycle plaster only as dry waste.

Wastewater: clay and glaze water must not go to drains untreated; use settling tanks, allow solids to settle, decant clear water, dry and dispose of sludge as solid waste.

Fired waste (shards, defective ware) can be crushed and used as grog or as aggregate; avoid sending to landfill where possible.

Energy efficiency: load kilns fully, fire during off-peak hours, insulate kilns, maintain burners and elements, and use waste heat for drying rooms.

Chemical storage: keep acids, fluxes, and colourants in labelled, closed containers on shelves below eye level; keep a safety data sheet (SDS) for every chemical.

Environmental compliance: follow local pollution control board norms for kiln emissions (particulate matter, sulphur oxides, fluoride from clay firing).