http://www.noteaccess.com/Texts/Cennini/index.htm
While his name reminds me of top hats with rabbits and waist coats, Cennino Cennini was actually an Italian man who recorded a great many construction processes during the late middle ages.
Here is what Cennini has to say about making crests:
'CLXIX How to Model Crests or Helmets.[211]
Whenever you have occasion to make a crest or helmet for a tourney, or for rulers who have to march in state, you must first get some white leather which is not dressed except with myrtle or ciefalonia,[212] stretch it, and draw your crest the way you want it made. And draw [p. 108] two of them, and sew them together; but leave it open enough on one side so that you can put sand into it; and press it with a little stick until it is all quite full. When you have done this, put it in the sun for several days. When it is quite dry, take the sand out of it . Then take some of the regular size for gessoing, and size it two or three times. Then take some gesso grosso ground with size, and mix in some beaten tow, and get it stiff, like a batter; and put on this gesso, and rough it in, giving it any shape of man, or beast, or bird, which you may have to make, getting it as like as you can. This done, take some gesso grosso ground with size, liquid and flowing, on a brush, and you lay it three or four times over this crest with a brush. Then, when it is quite dry, scrape it and smooth it down, just as you do when you work on panel. Then, in the same way, as I showed you how to gesso with gesso sottile on panel, in that same way gesso this crest. When it is dry, scrape it and smooth it down; and then if it is necessary to make the eyes of glass, put them in with the gesso for modeling;[213] do modeling if it is called for. Then, if it is to be gold or silver, lay some bole, just as on panel; and follow the same method in every detail, and the same for the painting, varnishing it in the usual way. '
So, he refers to other parts of his writing that tell a student about
* 'the regular size for gessoing'
* 'gesso grosso'
* 'gesso sottile'
* 'bole, just as on panel'
* 'varnishing it in the usual way'
There must be information on these processes in other portions of his text. I will get a-searching.
The phrase 'take some of the regular size for gessoing, and size it two or three times' confuses me. Does the second use of the word 'size' refer to a volume measurement? As in, take some powdered glue and divide/multiply it two or three times. Could that be it? I am still deciphering how the other use of the word size, meaning a glue, often bought as flakes made from rabbit hide.
Or does the text mean 'take some of the regular size...' [the glue you usually use] 'and size [glue] it [the crest model] two or three times' [with two or three layers of glue]
In the section marked 'CLXXIV How to Gild a Stone Figure', Cennini says
'take gesso grosso and size, tempered in the same way you gesso the flat of a panel or ancona, except that I want you to put in, according to the quantity, one or two or three egg yolks; and then lay it over the job with a slice... And apply this gesso two or three times with a slice, and let it dry out thoroughly.
When it is perfectly dry, scrape it and clean it up, just as you do on panel or ancona. Then take gesso sottile or gilders' gesso,[244] and temper and grind this gesso with the same size, just as you do for gesso on panel, except that you must put in a certain amount of egg yolk, not so much as you put into the gesso grosso...'
This implies that that gesso grosso is the coarser, rougher first layer of plaster that is used to make the basic form, while gesso sottile/guilders gesso is the finer or maybe more desirable coloured plaster for the outer layer. The text on making a crest doesn't specifically say to use gesso sottile on crests, however. It appears that he says to mix the gesso plaster and powdered size (glue), but only add egg yolks if forming the gesso over stone, not for forming over a leather base.
The part below of Cennini's text further explains gesso grosso and gesso settile.
'How the Flat or a Panel Should be Gessoed With the Slice with Gesso Grosso.
Chapter CXV
When the ancona is quite dry, take a tip of a knife shaped like a spatula, so that it will scrape well; and go over the flat. If you find any little lump, or seam of any sort, remove it. Then take some gesso grosso, that is, plaster of Paris,[109] which has been purified[110] and sifted [p. 70] like flour. Put a little porringerful on the porphyry slab, and grind it with this size very vigorously, as if it were a color...'
On re-reading this, Cennini is saying to take some dry flakes of the size and some dry plaster and grind them together, as you might when making a paint from ground pigment. Woo! I can do that! I am going to extrapolate further, that the purifying he talks of is some sort of process to remove any impurities like grains of rock or sand. Perhaps means running it through a rough mesh or picking out by hand, before using a fine sifter, like you would with flour. I have not seen metal wire mesh or a sifter from the medieval age. Some cultures use reed mesh in cane frames today, or open weave fabrics pulled taught over a frame or bucket to sift.
'How to Make the Gesso Sottile or Gessoing Panels.
Chapter CXVI
Now you have to have a gesso which is called gesso sottile; and it is some of this same gesso, but it is purified for a whole month by being soaked in a bucket. Stir up the water every day, so that it practically rots away, and every ray of heat goes out of it, and it will come out as soft as silk. Then the water is poured off, and it is made up into loaves, and allowed to dry; and then this gesso is sold to us painters by the apothecaries. And this gesso is used for gessoing, for gilding, for doing reliefs, and making handsome things.'
So that just leaves the question - what is 'beaten tow'?
Plenty to go on with anyway. I will keep digging.
