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Chemical structure of Iron Oxides
PBr7	PY43	PR102
PBr6	PY42	PR101

Chemical structure of Iron Oxides

The Color Index Number identifies the general chemical structure of the pigment. In many cases, slight variations of this chemical structure can cause dramatic changes in hue, although the C.I.# remains the same. This is particularly the case with the iron oxides (particularly yellow, red, and brown earths).

“Natural iron oxides” , also called natural earth pigments,

are clay (hydrated aluminium silicate) colored principally by iron oxides and incidentally by other compounds like manganese salts. They can contain various impurities like silica, alumina, organic matters, etc. Pure clay (kaolin) is uncolored and transparent when dispersed in oil.

“Synthetic iron oxides”

can be hydrated or anhydrous. They can be pure or fixed on a transparent substratum for making a large variety of pigments.

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PBr7

The chemical structure of PBr7 is natural iron oxide, in other words “natural earth of which the coloring matter is above all iron oxide”. But because the letters “Br” in the C.I.# mean brown, PBr7 means natural “brown” iron oxide. The various possibilities are:

C.I.#	Chemical structure	Common name	Color
PBr7	natural brown iron oxide	Raw Sienna
PBr7	calcinated natural brown iron oxide	Burnt Sienna
PBr7	natural brown iron oxide containing manganese	Raw Umber
PBr7	calcinated natural brown iron oxide containing manganese	Burnt Umber

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But it isn’t that simple. In any of these 4 variants, there are several hues, depending not only of the size of the pigment particles, but also on the quantity of manganese and other impurities it may contain (e.g. alumina, silicates).

For example, Burnt Sienna can be more or less yellowish or reddish, depending on the manufacturer.

Burnt Sienna, sort No.1
Burnt Sienna, sort No.2
Burnt Sienna, sort No.3

Another problem

Raw Sienna is PBr7 if you consider it’s a brown pigment. But if you consider it’s a yellow one, it becomes PY43. And indeed, if you place it next to brown, it looks like a yellow ochre, but if you place it next to yellow, it looks like a pale brown earth!

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PY43

Logically, the chemical structure of PY43 is natural yellow iron oxide. Here too, there are several possibilities, owing to the different impurity levels of the original earths:

C.I.#	Chemical structure	Common name	Color
PY43	natural yellow iron oxide	Yellow Ochre Light
PY43	natural yellow iron oxide	Gold Ochre
PY43	natural yellow iron oxide	Raw Sienna
PY43	natural yellow iron oxide	Yellow Ochre Deep
(PY43)	(calcinated natural yellow iron oxide)	(Red Ochre)

This is the theory, but in the practice, PY43 (calcinated natural yellow iron oxide) doesn’t exist, because this pigment is obviously red. Il will be PR102 (calcinated natural red iron oxide), in spite of the fact that it is in reality a calcinated yellow ochre!

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PR102

The chemical structure of PR102 is natural red iron oxide.

C.I.#	Chemical structure	Common name	Color
(PR102)	(natural red iron oxide)	(Yellow Ochre)
PR102	calcinated natural red iron oxide	Red Ochre

Here too, PR102 (natural red iron oxide) doesn’t exist, because it is a yellow pigment PY43.

Furthermore, the C.I.# PR102 (as PY43 and PBr7) will soon disappear completely, owing to the fact that most manufacturers gradually replace all their natural iron oxide pigments by synthetic ones. The same thing is true for all the other natural earths. By many brands, the labels PY43, PR102 or PBr7 don’t match natural pigments any longer, but various mixtures of synthetic iron oxides and should be labelled PY42, PR101, PBr6, etc. They seem to have kept the old C.I. numbers for commercial reasons only.

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PR101

The chemical structure of PR101 is synthetic red iron oxide. Some of the various possibilities are:

C.I.#	Chemical structure	Common name	Color
PR101	synthetic red iron oxide	Light Red
PR101	synthetic red iron oxide	Venetian Red
PR101	synthetic red iron oxide	Indian Red
PR101	synthetic red iron oxide	Caput Mortuum

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PY42

The chemical structure of PY42 is synthetic yellow iron oxide. Here are some brand names of this pigment, of which the hue can vary considerably.

C.I.#	Chemical structure	Common name	Color
PY42	synthetic yellow iron oxide	Yellow Ochre Light
PY42	synthetic yellow iron oxide	Gold Ochre
PY42	synthetic yellow iron oxide	Mars Yellow
PY42	synthetic yellow iron oxide	Transparent Yellow Oxide

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PBr6

The chemical structure of PBr6 is synthetic brown iron oxide. When very dark, this pigment can be an excellent substitute for Burnt Umber PBr7.

C.I.#	Chemical structure	Common name	Color
PBr6	synthetic brown iron oxide	Mars Brown, sort No. 1
PBr6	synthetic brown iron oxide	Mars Brown, sort No. 2

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