Fugitive Ink Question - Queen Victoria Penny Red & Penny Black

Recently in rereading Seymour's The Postage Stamps of Great Britain, I came across the section re: use of fugitive ink for these issues. So now a stamps 101 question: if so, why can these be soaked with no issues? I've soaked a few blacks and reds from time to time and noticed no color run…..

I thought that if stamps are printed with fugitive ink , the color will run when soaked??

There are two types of fugitiveness: water soluble and chemical soluble.

The concern the Post Office had was that the cancellation would be removed so the stamp could be reused. This required chemical removal.
In 1881, postage stamps became valid for revenue collection and certain revenue stamps became valid for postal use. The Inland Revenue was more paranoia and required stamps printed in double-fugitive inks, especially the most used denominations. At the time, lilac and green were double-fugitive inks. That is why these two colours were used so frequently from 1881 onwards.

Interesting the few articles I read do not distinguish- water vs chemical , single or double. Does make sense seeing as reds and black can both be soaked.

So is single Chemical Fugitive and double Chemical & Water?

There is fugitive and double-fugitive. The latter refers is used when the ink washes when treated with water or chemicals (usually oil-based) agents. I suppose 'single' fugitive, could refer to either. In practice, it might turn out to wash when treated with chemicals.