Silk Paper

I'm new to revenues. When did collectors start paying attention to whether or not the stamp was on silk paper?

I ask because I have a Harris catalogue from only two or three years ago. and it gives prices for a, b, and c stamps, but no d. I have a book abt revenue stamps from the '50s and it devotes two paragraphs to silk paper, doesn't say a word about how to detect it, and says nothing about the effect on the value of the stamp.

I've also bought some stamps labeled c, and when I looked the back under my cheap 10 x magnifier, there's a blue thread.

Don

Scott has listed the silk papers for decades. The problem is the catalog you are using.

The Boston Book of 1898 discusses silk papers, and there were articles before that in various philatelic magazines, so they have pretty much always been collected. Harris probably didn't mention them to save space, most of their customers probably didn't care much about them.
Recognizing true silk paper stamps is one of the more difficult areas to learn on one's own. First issue paper is often pretty dirty, and has fibers and other flotsam that are often mistaken for silk threads. True silk threads are short, almost always straight, and are blue. They must be embedded in the paper and not simply laying on the surface. They were issued in 1870, so any stamp cancelled before that is not silk although it might have fibers left over from the papermaking process.