Terracotta Batch Matching
A student is mixing acrylic paint for a triptych of three 60 cm × 90 cm panels. The target is a muted terracotta that will sit next to unglazed red clay tiles. The first batch is 500 ml: 70% cadmium red (PR108), 20% burnt sienna (PBr7) and 10% titanium white (PW6), measured by volume. After 24 hours the dried panels look about one Munsell value step lighter and noticeably pinker than the wet sample. A second batch mixed to the same volume ratios looks different again - slightly more orange and less chromatic. The deadline allows one more batch. The student has a scale, a palette knife, and can make drawdowns on the same primed board.
1. Diagnose why the dried paint shifted lighter and pinker, and why the second batch differed.
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Acrylic paint dries lighter because the wet film contains water and binder that scatter light; as the film consolidates, the pigment particles pack and the surface changes. Titanium white is the likely main cause of the pink shift: PW6 has very high tinting strength and high scattering power, so 10% by volume is enough to raise the value and cut the chroma of the red/sienna blend, pushing a warm terracotta toward a chalky pink. The second batch differed because volume is a poor measure for pigment pastes: cadmium red and burnt sienna have different densities and settle at different rates, so 70:20:10 by volume does not deliver the same pigment ratio each time. If the sienna was not stirred fully, the second batch would be more orange and less chromatic, exactly as described. The 24-hour dry-down also means the student was matching the next batch to a wet sample that no longer represented the dry target.
2. Give a corrected procedure for the final batch, including what to measure and how to test it.
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Make a weighed base, not a volume mix. Stir each jar thoroughly, then weigh a 200 g trial: 140 g cadmium red, 40 g burnt sienna, 20 g titanium white. Mix the red and sienna first, then add white in 2 g increments, recording each addition. Make a drawdown on the same primed board for each increment and dry it with a hair dryer on cool or wait; compare the dry drawdowns under the same light and against the clay tile. Choose the sample that matches in value first, then chroma. If the mix is too pink, reduce white and add a little burnt umber or yellow ochre rather than more red; if it is too orange, add a trace of a blue-green complement. For the 500 ml final batch, scale the chosen weighed ratio by the same factor and keep a labelled wet sample and a dried coupon. This gives a repeatable recipe and checks the wet-to-dry shift before the panels are painted.
3. The student wants to add black to darken the mix. Evaluate and propose an alternative.
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Black is a weak choice here. A black pigment lowers value but also lowers chroma and often shifts the hue toward neutral or blue-grey, so a terracotta can turn ashy and lose its clay relationship. In a three-panel set, that shift will be very visible beside unglazed tiles. If the mix needs to be darker, use burnt umber or a darker red iron oxide, which lower value while keeping the warm hue family. A tiny amount of the complement (a blue-green) can mute the mix without greying it as flatly as black. The exception is a very small amount of black used to adjust a too-bright orange, but it should be less than 1% and tested in a drawdown. The decision should be made on a dried sample, not the wet mix.