There are a few types of pencils that are considered to be (mostly) obsolete. Amongst these are electrographic pencils, non-photo blue pencils, stenographers pencils and especially indelible pencils--also known as copying pencils.
Copying pencils date back to to the late 1800s and were made for copying documents. The pencil contains graphite and clay, as a normal pencil does but also aniline dye that gives it permanence and also allows it to be transferrable. The way it works is that you take a document written in copying pencils and use a press to transfer it onto a sheet of tissue, often in a copying book. They were an essential tool for any business in the late 1800s/early 1900s and also rose to popularity during WWI as permanent, portable, easy-to-use writing instruments that were simpler to use than the fountain pens that were otherwise considered the norm at time. Shortly after the war was over the ballpoint pen was invented and largely overshadowed the purpose of having an "ink" pencil. Copying pencils were still used through the first half of the 20th century but slowly diminished as better, more efficient copying techniques were developed.
A few contemporary brands make versions of a copying pencil , though they're significantly less effective than their predecessors, mostly because they don't contain the toxic aniline dyes that they once did. Vintage ones are easy to find on the internet and come in various shades of blues and purples, and sometimes red. They're almost always of a medium or firm hardness, so as to make a sharp, legible mark. As for the color--they were originally purple because they were make with mauveine, the first discovered organic chemical dye. Named after William Henry Perkin, who discovered the compound, Perkin's Mauve is a rich, luminous, slightly red-ish hue.
Even as a 21st century gal, I use indelible pencils often. They're excellent for signing things (I've been using an Eberhard Faber NoBlot to sign copies of my book) and also for writing letters as they don't smudge like a regular writing pencil would (by the way--did you know that April is National Letter Writing month?!). If you want to try to use them for their copying abilities, go for it--just be prepared to work for that copy.
Photo: Some copying pencil packaging from our archives.