Indian yellow

Indian yellow is a natural dye that was used for centuries in the South Asian painting. Chemically, it is a laked xanthene Glucoronosid (see laking ). The dye is no longer commercially available.

Origin and Production

Genuine Indian yellow is a product of animal metabolism. You won it from the urine of Indian cows. These were fed with reduced fluid intake with mango leaves ( Mangifera indica, Anacardiaceae ). Because pathological metabolic processes and nutrient deficiency, the animals excreted from an intensely colored urine. One concentrated on these by heating, with farewell the yellow dye. This was pressed and formed into balls. In the trade, the Indian yellow from the East Indies then came in the form of large balls, called Piuri ( bengali: পিউরি ). The yield was about 50 g per day and cow Piuri.

Arrangements of the Indian government prohibited the end of the nineteenth century, the cows are fed with mango leaves. With the beginning of the 20th century, the dye disappeared from the market for animal welfare reasons.

Ingredients of Piuri

Indian Yellow dye is a magnesium / calcium Euxanthinsäure the color coat. Euxanthinsäure is a glycoside of glucuronic acid and Euxanthon ( = 1,7- dihydroxy -9H -xanthene -9-one ). It thus belongs to the series of xanthene dyes. Good qualities of Piuri contained ≈ 65 % color paint, bad only ≈ 35%. In bad qualities predominantly lay before the hydrolyzate Euxanthon.

Use

The genuine Indian Yellow was historically used primarily as a watercolor; but it also found in oil painting application as a glaze color. It was in the time of Mughal period ( 16th to 19th century) is a popular yellow pigment of the art painting in India. It appreciable quantities were exported to Europe; yet there are few sources for use in European art painting. It was first detected in Vermeer's "Woman with scales ". Contrast could be detected in many Asian paper paintings from the same period Indian yellow.

Proof

The simplest indication of the use of Indian yellow is obtained by viewing the art object in the UV light. Inspired with long-wave UV light ( 365 nm ) are presented in an intensely - yellow fluorescence. Thus, the use of Indian yellow in yellow, green and orange shades in South Asia, especially India, demonstrated miniature paintings from the period before and around 1900. In more recent miniatures these typical fluorescence missing. Further evidence can be by detection of Mg2 with quinalizarin obtained ( detection limit <1 ng). Final proof is done by means of IR spectroscopy.

Coloristics

Indian Yellow produces a dark, deep warm reddish yellow. The original can not be displayed due to the high color saturation in the monitor gamut. It is redder than the natural Siena, but much deeper color, applied thickly it looks brownish. The color is between buttercup yellow and saffron yellow.

Replacement dyes

Today, the genuine Indian Yellow largely by synthetic dyes from the class of azo dyes / Indanthrengelb ® from BASF or by cobalt yellow ( see Kaliumhexanitrocobaltat ) is replaced. A natural source for a dye appropriate Nuance is the luteolin -containing dye plant Wau ( Reseda luteola ).

Spurious Indian yellow

As for authenticity Indian yellow a nitration product of tropeolin is called.