page forty-five

The Vat

Forty-four rooms, and every colour in them was simply chosen. Today's number sets up a dye-house in a Georgian cloth town and a draper sends in a snip of cloth to be matched. Choose a mordant, weigh madder, weld, indigo and the rest into the pot, and dip test skeins until you dare dye the piece. It has to hold by daylight and by candle.

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The mordant

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The cupboard, weighed to the pound of wool

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The drying pole: pattern and test, by day then by candle
    Vat
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    The arrow keys walk to other days. Every day is a different house, a different cupboard and a different pattern.

    What is real here and what is not

    The dyestuffs, the mordants and the colour science are real. The dye-houses, the drapers and the exact absorption of each dye are the day's number and a simplification.

    1. 01

      A cupboard of plants and insects

      Before the aniline dyes of the 1850s, every colour on wool came from a short list: madder root and cochineal for reds, weld, fustic and quercitron bark for yellows, logwood for purples and blacks, and the indigo vat for blue. Greens, browns, purples and greys were all mixtures. A dyer matched a pattern by eye, from a book of recipes and a lifetime of test skeins.

    2. 02

      The mordant changes the colour

      Most natural dyes will not hold on wool without a metal salt to bind them, and the metal alters the colour. Alum leaves it clear. Tin, used after Cornelis Drebbel's discovery around 1607, turned cochineal from crimson to a blazing scarlet. Iron, as copperas, "saddens": madder goes to plum and brown, yellows to olive, logwood to black. Indigo is different: it is reduced in the vat, soaks in, and turns blue in the air, so it needs no mordant and deepens with every dip.

    3. 03

      How the colour is computed

      Each dye here has an absorption spectrum made of a few bands, which the mordant shifts, widens or greys. In the pot they add up as K/S, the absorption over scattering of Kubelka and Munk's 1931 theory, which gives the cloth's reflectance at every 10 nm from 400 to 700. That curve is lit by CIE daylight D65 or by a black body at 1850 K for the candle, seen through the CIE 1931 observer, and compared with the pattern in CIELAB by the CIEDE2000 formula. A ΔE near 1 is about the smallest difference a trained eye sees side by side.

    4. 04

      Metamerism

      The eye squeezes a whole curve down to three numbers, so two different curves can give the same three in one light and different ones in another. Dyers knew it long before anyone could explain it, and matched patterns at a north window and again by candle. Shop lighting still catches people out today. When the piece is dyed, the page shows the curves themselves, which no Georgian dyer ever saw, and a rival house's match that may hold by day and fail by candle.

    Forty-five rooms, and one number doing all the work. Today it sets up a dye-house, stocks its cupboard and has a draper send in a pattern that only one pot of dye can match in every light.