Interesting Cennini references

http://www.noteaccess.com/Texts/Cennini/index.htm

Section one
Chapter XVI

 "get a leaf of druggists' glue, not fish glue,"

Chapter XXXI

"When you have got this done, take a little white lead well worked up with gum Arabic."

Section three

"Then, when you want to work, remember first to make this plaster quite uneven and fairly rough."

Section 5

"There is one size which is made of cooked batter, and it is good for parchment workers and masters who make books; and it is good for pasting parchments together, and also for fastening tin to parchment. We sometimes need it for pasting up parchments to make stencils. This size is made as follows. Take a pipkin almost full of clear water; get it quite hot. When it is about to boil, take some well-sifted flour; put it into the pipkin little by little, stirring constantly with a stick or a spoon. Let it boil, and do not get it too thick. Take it out; put it into a porringer. If you want to keep it from going bad, put in some salt; and so use it when you need it"

Section 5

How Goat Glue is Made, and How it is Tempered; And How many Purposes it will Serve.
Chapter CVIIII
"And there is a glue which is known as leaf glue; this is made out of clippings of goats' muzzles, feet, sinews, and many clippings of skins. This glue is made in March or January, during those strong frosts or winds; and it is boiled with clear water until it is reduced to less than a half.[102] Then put it into certain flat dishes, like jelly molds or basins, straining it thoroughly. Let it stand overnight. Then, in the morning, cut it with a knife into slices like bread; put it on a mat to dry in the wind, out of the sunlight; and an ideal glue will result. This glue is used by painters, by saddlers, and by ever so many masters, as I shall show you later on. And it is a good glue for wood, and for many things. We shall discuss it thoroughly, showing what it may be used for, and how, for gessos, for tempering colors, making lutes, tarsias, fastening pieces of wood and foliage ornament together, tempering gessos, doing raised gessos; and it is good for many things."

A Perfect Size for Tempering Gessos for Anconas or Panels.
Chapter CX
"And there is a size which is made of the necks[103] of goat and sheep parchments, and clippings of these parchments; these are washed [p. 67] thoroughly, and put to soak a day before you put them on to boil. Boil it with clear water until the three parts are reduced to one. And when you have no leaf glue, I want you just to use this size for gessoing panels or anconas; for you cannot get any better one anywhere."


A Size Which is Good for Tempering Blues and Other Colors.
Chapter CXI
"And there is a size which is made from the scrapings of goat or sheep parchment. Boil them with clear water until it is reduced to a third.[104] Know that it is a very clear[105] size, which looks like crystal. It is good for tempering dark blues. And apply a coat of this size in any place where you have happened to lay in colors which were not tempered sufficiently, and it will retemper the colors, and reinforce them, so that you may varnish them at will, if they are on panel; and blues on a wall the same way. And it would be good for tempering gessos, too; but it is lean in character, and it ought to be rather fat for any gesso which has to take gilding."

Section 5
To Make a Glue out of Lime and Cheese. 
Chapter C XII
"Ends the Fifth[107] Section of This Book.
There is a glue used by workers in wood; this is made of cheese. After putting it to soak in water, work it over with a little quicklime, using a little board with both hands. Put it between the boards; it joins them and fastens them together well. And let this suffice you for the making of various kinds of glue."

Section 6

on using sheep size, 

"any sort of work which you have to gesso, using a large soft bristle brush. 

Then let it dry. [p. 69] Next take some of your original strong size, and put two coats over this work with your brush; and always let it dry between one coat and the next; and it will come out perfectly sized."

How You May Gesso with Gesso Sottile Without Having Gessoed with Gesso Grosso First.
Chapter CXVIII
"Furthermore, it is all right to give any small-sized and choice bits of work two or three coats of size, as I told you before; and simply put on as many coats of gesso sottile as you find by experience are needed."

Leather Helm Crest: Part 10 "...size it..."

After rehydrating the hide glue, it was left for a day. This is how I prepared the crest for gesso by sizing it (painting it with size/glue).

I took the lid off the jar of size and added hot water from the kettle to a pot. I tried to get you a nice pic of the set up but the camera fogged up so my kitchen looks like a 70s family photo. I had intended to bring the water temperature up on the stove but the left over water in the kettle, after pouring enough for a cup of tea, was enough heat to melt the hide glue. The open jar was sat on a small ceramic plate, to act as a trivet and keep direct heat from the base, if the water needed to be heated on the stove.

After 5 minutes or so, the glue had melted.

The leather crest had the dried sand removed from the inside, but each time I moved it there seemed to be more. I will really have to give it another solid shake and brush before the final coat of paint or the sand will find a way to get on the paint and make it look grainy.

 I applied one coat of size and left it to dry for an hour. When I returned, it was touch dry with a gel-like texture. You can see, in the jar, how much size it took for a single coat coverage. If around a 1/3 was used in a coat, and the whole volume took 3 teaspoons of dried hide glue, it took around 1 teaspoon for the first coat.

After an hour the size had reset. This time, placed the open jar of size back in the water bath and turned the stove on to the lowest setting. After a few minutes the glue had started to melt. After 5 minutes, the trivet started to bounce around and the glue began to smell. At this point, it had become a golden honey colour and texture. The glass bottle was still not very hot, as I could pick it up with bare hands.

I applied a second coat of size, making sure to brush in a different direction. You can see the brush strokes at 90 degrees to each other on the snout. Leave the size to dry and then you can cover it in gesso grosso.

Leather Helm Crest: Part 7 Mixed hide glue

In preparation for applying gesso sottile over a layer of gesso grosso, I have mixed up some hide glue.

Put 1 part powdered hide glue and 2 parts water into a jar. The picture below shows the water and powder mixed on the left, sitting next to a jar of dry hide glue powder on the right

 

Let it soak for an hour, to take up the water. 

It becomes much thicker and more rigid, on soaking up the water.

To use this glue, it is recommended that you stir the granules and leave them overnight. Then, you can warm the glue up to a liquid state, using a glue pot or gentle heat in a double boiler. In a sealed jar, this glue can last a couple of weeks, at least. It is water soluble and can be reheated to return it to a liquid state, even when on a project.  

 This is what the rehydrated glue looks like, before being heated to a liquid state. The volume shown is 3 teaspoons of dry size and 6 teaspoons of water. It is the same volume in the picture to the left and the jar on the left in the image above. The expanded size has been stirred and chopped up with the plastic knife shown on the left, making it fluffier. Hence, it appears larger. 

This will be heated and painted on to the leather crest shape in 2 - 3 layers before the gesso layers are added.

Hide glue how-to

http://www.stewmac.com/shop/Glues,_adhesives/Wood_glue/Behlen_Ground_Hide_Glue.html?tab=Video

Here is a video showing how to make up hide glue. It follows a process I have seen in a few other places. You can also buy hide glue from the online shop. Luthier suppliers are a good place to get hide glue, which comes in amber coloured flakes.

Gesso from calcium carbonate vs gesso from calcium sulphate

In my last post, I briefly outlined what gesso was. I am still looking into how it came to be used and why type was chosen over the other. I used the websites listed in the previous post, particularly this e-book, and will refer to it again here.

Calcium Carbonate
Calcium carbonate was typically used in Northern Europe, most likely because this is where significant and accessible deposits were being mined during the middle ages. It is also known as chalk. Chalk is the remains of ancient sea creatures. It can be finely ground and to make chalk whiting. This whiting can be turned into gesso by adding a size.

Calcium Sulphate
Bulian and Graystone say it was typically used around the Mediterranean region, most likely because Bologna and Volterra have large deposits. Jerry Tresser also records that in the 1300s there was a large deposit being mined at Montmartre, near Paris. This most likely accounts for the common name of the refined product: Plaster of Paris.

The mined substance is known as gypsum. This is CaSO4.2H2O [Calcium sulphate dihydrate]

Gypsum was mined, ground up and burned to remove water and any impurities. Tresser says this process also changes the natural gypsum from yellow to white. When this dried product was ground again, it has the adjust chemical composition CaSO4.1/2H2O . It was, and still is, sometimes called Plaster of Paris. This powder is called gesso in Italian.

Gesso was used in 2 ways.
1. Gesso Grosso
CaSO4.1/2H2O + Size
Plaster of Paris, or gesso, was sifted and then moisten it, assumedly with water.

Add parchment size or hide size to this paste to make gesso grosso. This is the first layer Cennini recommends to apply over the leather crest shape. It is fast drying, but this means bulky sculpture protrusions have less time to sag or drip. This makes it a good material for roughing out the shape.

2. Gesso Sottile
CaSO4.2H2O + Size
Plaster of Paris, or gesso, was soaked in water for several weeks
Cennini says to soak it for 30 days. Jerry Tresser points out that this is an imprecise process, as it is not possible to tell when the gesso was been super hydrated. After reading a few other versions of this process, I think Cennini likely chose 30 days as a safe, standard soaking time to ensure the process was completed by this point.

The water is drained off and the paste is set into blocks to dry again. This powder was called gesso di bologna.

Mix it with size to make Gesso Sottile. This is used in the fine finishing layer of Cennini's crest making process. It has the benefit of being slower drying, allowing more time for fine detailing.

What is Gesso , Ground and Size?

Reading Cennini's instructions on making gesso left me confused so I went searching. The best results I found were at Qualitative Study of Cennini's Gesso for Illumination and Wood Coatings: Theory and Practice pp.142-144 The book is available for purchase as a hard copy or e-book. I liked this person's profile and their enthusiasm for the modern version of the process. Here is a summary of my new understanding of the materials and process.

What is a Ground?

When timber or leather are prepared for painting, it is sometimes useful to place a layer of substance that will enhance the surface texture and the appearance of the paint. Gesso is one form of ground. It is a white substance that is painted over the main structure. In the case of crests, Cennini recommends building a structure of moulded leather and applying several layers of gesso as the ground.

What can Gesso be made from?

Sometimes texts simply use the term 'white ground', which is not very specific but is a good umbrella term. The short answer is that there were 2 main substances in use as white grounds, though other less common materials were also suitable.  

Most Common

Whiting    CaCO3 

AKA Calcium Carbonate

- This is available at my local Eckersleys art shop

Plaster of Paris   CaSO4.1/2H2O

Derived from calcium sulphate

*Note* There are several versions of the usable product so I will add detail

This is available at all hardware shops and art shops

Less Common

Kaolin         A2O3.2SiO2.2H2O
Essentially, a clay, but with a wide range of uses and preparations
Synonyms 
- This is available at my local Eckersleys art shop.

Dolomite     CaMg(CO3)2
Magnesium Calcium Carbonate
- I have not seen this for sale but have not really gone looking
*Note* The term dolomite seems to be a synonym for Calcium Carbonate, even though the chemical composition is different.

What is Size?

Size is another word for glue - a very simplified description, but it makes sense in most contexts. Rabbit hide glue, or just 'hide glue', is available online from book making and fine woodworking websites. It comes as orange brown crystal flakes. Cennini recommends making the best size from the "necks of goats and sheep by trimming, washing, soaking and then boiling it," (Thompson, 1960, cited in Bulian & Graystone, 2009, p. 143, linked above). I will have a look at Cennini for the exact quote. This book also mentions two other "historic treatises" by Theophilus and Watin. Both have a preference for animal glue but none mention rabbit skin glue (Souza and Derrick, 1995, cited in Bulian & Graystone, 2009, p. 143). This implies that perhaps we use rabbit hide glue for most of our reconstruction projects simply because it is readily available at the moment, rather than it is readily available because of superior properties.