You're right that it doesn't. However Crawford does make it clear that
what he is intending is a solution that is saturated at 20 degrees
Celsius.
One point of using a saturated solution is to avoid having to weight the
stuff out, so giving a recipe in terms of masses is pretty silly. Plenty
of great gum prints have been made by people who didn't have access to any
more sophisticated measuring system than a teaspoon.
Crawford made up the figures (perhaps at an editor's request?) and he got
this wrong. To get an accurately made saturated solution you just dissolve
the stuff, make sure there is excess solid, leave it lying around at 20
degrees for a while and then decant it to use.
For most of the solutions we use it is sufficiently accurate to assume
that the volume of the solution is the same as the volume of water used to
make it. So we get a 10% solution by adding 10g solid to 100ml water
(rather than adding to a smaller volume and making the solution to 100ml
as is normal chemical practice.) And if you always do it this way there is
no problem of course.
Peter Marshall
On Fixing Shadows and elsewhere:
http://www.people.virginia.edu/~ds8s
Family Pictures, German Indications, London demonstrations &
The Buildings of London etc: http://www.spelthorne.ac.uk/pm/