May 12, 2010, 05:15 PM
Bloom strength, also called gel strength or gram weight strength, correlates to the molecular weight of the glue. The higher the bloom strength the faster the glue will gel, the better its adhesive properties, and the more flexible the dried adhesive.
So, if you are preparing a ground composed of powdered solids (such as a chalk or gesso ground), a lower molecular weight glue will penetrate more effectively in both the solid powder and the porous substrate, and because low bloom strength glue gels more slowly, there is more time for the glue to penetrate before it becomes an immobile gel. Hence a balance between the qualities provided by low and high bloom strength glues need to be considered for the type of ground employed.
As with other glues derived from fish, isinglass has a different protein composition than animal collagen. Hence, isinglass and other fish glue do not have bloom strength, because they remain soft at room temperature. (Remember that bloom strength is measured by pressing a cylinder of known weight and size into a gel at room temperature.) The molecular chain length of fish collagen, however, can be compared to the best grades of animal glue, and hence isinglass can be a very strong adhesive. The viscosity of isinglass and other fish glue is not related to the chain length as is the case of animal derived collagen glue.
May 13, 2010, 11:54 AM
Thank you, George! You are wonderfully knowledgeable and generous with that knowledge.
By the way, I've noticed that a couple of purveyors to the calligraphy community sell calcium sulfate dihydroxide for raised gesso for gilding -- is it exactly the same as yours?
May 13, 2010, 01:08 PM
I cannot say that it is exactly the same thing, although chemically they are the same thing. The "proper" chemical name for gypsum or gesso (Italian word for gypsum) is calcium sulfate dihydrate, but the chemical name can describe many different varieties of the chemical all with different properties!
May 15, 2010, 12:21 PM
How does the alabaster compare to the Italian gesso for raised gilding work on paper or vellum? Would one be preferable?
Thank you again!
May 15, 2010, 05:10 PM
I think both alabaster and Italian gesso work quite well in raised gesso. I cannot say one would be more preferable over the other, except for some nuances. Start with the Italian gesso first.