>I do not understand why a substrate is thought to be needed to stabilise the
>paper.
The substrate not only aids in stabilising the paper for registration but it
allows for forced air drying with a hair dryer. Ordinary unsubstrated paper
will buckle, curl and puddle, resulting in an uneven splotched development
if a hair dryer is used. Forced air drying allows one to halt the
development of a short exposure and short developed print without which the
image would just "slide" off the print if just hung up to dry. Short
exposure/development allows one to quickly build up a multi coat print.
Typically a first coat might be lampblack to get a base image. Then followed
by several color coats building up to the desired density, say using sepia
or sienna as one of my students did with an image of St. Marks in Venice
taken at twilight. The final coat was a way underexposed coat of Winsor
Newton Blue Black, which developed for a short time and drained and was laid
flat and left for the blue black to "bleed" over the print, then quickly
halted at just the right point with two hair dryers. No way could this have
been accomplished with unstabilized paper.
Dick Sullivan
Bostick & Sullivan