Materials, tools and techniques: properties, handling and care
This domain tests whether you can name a material, explain how it behaves in the hand, and justify a choice for a specific job. PAU questions rarely ask only 'what is gouache?'; they ask which medium you would use for a given problem and why, so every property you quote must be tied to a practical consequence.
Supports and grounds
- Grammage (gsm) is weight, not quality: 90 gsm paper cockles under a wash, 300 gsm cotton rag absorbs wet-on-wet without buckling.
- Sizing (gelatine or alkaline) controls absorbency; unsized paper feathers ink and bleeds.
- Acid-free, buffered papers and boards resist yellowing; wood-pulp board embrittles and browns.
- Grounds: acrylic gesso for canvas and panel, traditional glue-and-chalk for tempera panels; a toned ground helps you judge values while painting.
Pigments, binders and diluents
- Paint is pigment plus binder: gum arabic (watercolour, gouache), egg yolk (tempera), linseed oil (oil), acrylic polymer dispersion (acrylic), wax (encaustic, oil pastel).
- Pigments differ in opacity, tinting strength, lightfastness and toxicity: cadmiums and lead whites are opaque and permanent but toxic; some reds and yellows are transparent and fade.
- Diluents: water for water-based media, turpentine or white spirit for oil. Thin acrylic with medium rather than excess water, or the polymer film loses integrity.
Tools: selection and care
- Kolinsky or sable keeps a point and spring for watercolour; hog bristle is stiff enough for oil impasto; squirrel holds a soft wash; synthetics suit acrylic and sharp edges.
- Shapes carry function: round for line, flat for blocks, filbert for rounded blending, fan for texture, mop for washes.
- Clean acrylic out before it cures, because a dry polymer film no longer re-wets; comb oil brushes and store them bristle-up; keep markers horizontal so both ends stay inked; use a kneaded eraser on graphite and never scrub charcoal.
Worked example
A three-hour still life with strong tonal contrast. Oil cannot be layered and dried inside the session; watercolour struggles to restore clean highlights once the paper is stained. Gouache on 300 gsm cold-press board answers the brief: body colour is opaque enough to place a light shape over a dark one, errors re-wet and blot away, and the matt surface dries quickly. The reasoning is opacity, re-wettability and drying time, not personal preference.
Traps that cost marks
- Assuming a heavy paper is automatically sized for wet media.
- Spraying fixative in thick clouds, flattening charcoal and dulling pastel.
- Thinning acrylic with too much water and calling the result a wash.
- Leaving natural brushes standing bristle-down in water, which curls the hairs permanently.