While red earth is perhaps the oldest pigment, PR101 refers to the synthetic earths, while PR102 is for naturally occurring red earths. Red iron oxide pigments are useful in mixtures to complement blues, to make a range of flesh tones, and to create deep dark passages at low lightnesses or to create glowing chroma.

The names for synthetic red earths often mirror the natural red earth names, and they were often named for places where the pigments were historically mined. Each place is associated with a distinctive set of qualities, and these traits helped to form the expectations surrounding the traditional paint names (e.g. Venetian Red). A cannon has arisen around these names such that synthetic colors with similar properties may borrow from the geographic names. Unfortunately this is not a guarantee as there is no official standardization. One name that is often associated with PR101 in particular is Mars Red, which is often a powerful dull red color. Another frequently found name is Transparent Red Oxide, which is usually deep in masstone and reddish brown in glazes.

PR101 includes a huge variety of synthetically made red earths, oranges and violet-browns. The particle sizes can vary considerably, and with the synthetic version it can sometimes be very small- though this is by no means a guarantee. Some brands, like Williamsburg, publish guides to the graininess of their colors, while other brands rely on the brand vibe to convey the degree of homogenization among their colors (for example, some brands are known to make very smooth colors overall).

While generally thought to be among the safer pigments, it is at least theoretically possible that some synthetic earths could be derived from reclaimed mining sites, so we always just handle them like all paints with proper studio procedures. The Artist's Guide to Health and Safety has more information about health issues associated with pigments which contain iron. Her updated work can be found through her site.

When it comes to color, the sub-variants of PR101 are a wide-ranging bunch. Interestingly "Red Iron Oxide" can be red, brown, orange, or violet. They are also incredibly lightfast and durable.