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 8 Tow

In returning to an old question I had asked about tow, Cennini has the following to say on preparing gesso grosso:

'CLXIX How to Model Crests or Helmets.[211]
(after making the leather base)... 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.

1.    "...regular size for gessoing..."   As discussed in an earlier post, rabbit hide glue was not listed as the most common in use during this time period. It is, however, the most readily available form now. I will use it for this project and investigate other alternatives for future projects, to compare results.

2.    "...size it two or three times..." This could mean 'apply hide glue size to the outside of the leather base in two or three coats.' We often apply a coat of glue as a sealer to woodwork or leatherwork projects, to give better coverage of outer coats of paint. This may help to attach the plaster layer.

3.   "...Then take some gesso grosso ground with size..." We are going to use commercially available Plaster of Paris for our gesso grosso base. Since our plaster comes pre-ground in powdered form, rather than in blocks like Cennini had to deal with, we can go right ahead and mix in some powdered hide glue. He does not provide quantities or ratios. The text only implies that the size proportion is smaller than the plaster proportion.

4.   "...and mix in some beaten tow..."   I have found a few relevant descriptions for tow.

The Free Dictionary website says 

"n. Coarse broken flax or hemp fiber prepared for spinning.... ...[Middle English, possibly from Old English tow-, spinning (in tow-cræft, spinning craft, spinning)."

Word Reference.com says 

"tow /təʊ/    N

1. the fibres of hemp, flax, jute, etc, in the scutched state
2. synthetic fibres preparatory to spinning

Etymology: Old English tōw; related to Old Saxon tou, Old Norse tō tuft of wool, Dutch touwen to spin"

Dictionary.com says

"tow

2  [toh]  noun

1.

the fiber of flax, hemp, or jute prepared for spinning by scutching.

2.

the shorter, less desirable flax fibers separated from line fibers in hackling...

...Origin:  1300-1350; Middle English; 

Old English tōw-  (in tōwlīc  pertaining to thread, tōwhūs  spinning house); 

akin to Old Norse tō  wool

tow

3  [toh] noun Scot.

a rope.

Origin: 1425-1475; Late English (Scots);

Old English toh-  (in tohlīne  towline); cognate with Old Norse tog towline."

This suggests that if I take some rope made from the most common material of 1440, being either flax, hemp or jute, and cut very small fibres up to mix into the plaster, I can use this to "...mix in some beaten tow..." I suppose it would be possible to use unspun flax or jute from another source, too.

This method is supported by the composition of an extant bird crest. This bird is listed as being "...constructed of gesso and coarse fibre, probably jute, over a hardened sculpted leather core..." 
http://www.invaluable.com/auction-lot/cimier-heraldique-pour-grand-heaume-de-tournoi,-40-c-ee0776023c

So, I shall mix some finely chopped jute rope and powdered hide glue into some Plaster of Paris to make gesso grosso, and then paint the thick paste on to the leather form. It finally makes sense!

Extant bird crest, sans wings

The website lists this crest as sold. I have found items on auction websites before, only to lose the information when the next lot is listed, so I aim to preserve the text here. The website lists...
 "A German heraldic crest for a tournament Great helm, Zimier, in 14th-15th century style. 

Formed as a Swan's head, accurately constructed of gesso and coarse fibre, probably jute, over a hardened sculpted leather core, the base hollowed for fitting the crown of the helmet skull, with pairs of lace-holes at the sides, painted white and heightened in grey, the base and the beak with traces of gilding over a red base coat, and in "aged " condition throughout. 
H. : 37 cm - Wt. : 1095 g"
http://www.invaluable.com/auction-lot/cimier-heraldique-pour-grand-heaume-de-tournoi,-40-c-ee0776023c

Points to note
* The lower edge is gently scalloped.
* There are two lacing points on the lower side. They are spaced at a ratio of 1:2:1 for hole:space:hole. They are a little over one hole diameter above the bottom edge. It would be a reasonable conclusion to expect another pair of holes on the other side.
*Based on the image size and the described height, the holes are around one centimetre in diameter, and the base is near to 22 cm in diameter.
* the lower edge does not appear to have an internal dome/cap.
*The lower edge has a 3.5cm wide band of smooth finish - potentially a thicker leather band to provide rigid support.
* The eyes are sculpted, while the feather detail is entirely painted on.
* The neck, at it's narrowest point, is 6 cm diameter.

I am getting a better idea of how this crest is going to look.

Extant Crest: The Black Prince

When Edward III died in 1376, several items of heraldic display were arranged adorning his tomb. Among them, is a crest. It is a leather formed leopard, covered in gesso and gilt. This is one of the extant items I looked at when planning how to make

Here is the website that gives a bit more history and shows a reproduction.
http://www.myarmoury.com/feature_mow_bp.html

Here is a discussion where the author of the above page shows his reproduction crest and talks with Armour Archive members about armour mirroring contemporary fashions and people's interest in wearing crests in combat now.
http://www.myarmoury.com/talk/viewtopic.php?t=5265

This page shows some photographs of the original items in the wall case of Canterbury Cathedral.
http://piersperrotgaveston.blogspot.com.au/2011/01/canterbury-cathedral.html

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.

Leather helm crest: Part 6 Preparing gesso sottile

About a week ago, I started to saturate some plaster of Paris to make gesso sottile. This is the second layer of gesso that Cennino Cennini recommends for the outside of a helm crest. In an earlier post I explored the chemical composition of gesso. Gesso sottile is a super hydrated version of plain plaster of Paris. By soaking the plaster, you are allowing it to take on more H2O (water), giving it the desirable property of a slower drying time. This means we will be able to add the final sculpting touches with more care.

Since this is the first time I have used plaster in this way, to sculpt rather than to make moulded objects, it is all a learning experience. I have no idea how well the plaster will stick to the leather, or how well the second layer of plaster will stick to the first, or how the paint will need to be applied. It will also be the first time I have made pigment based medieval paints. There will have to be some test pieces very soon...

Cennini suggests leaving your plaster to soak for 30 days, changing the water to ensure it remains clean. Other sources say this method is inexact - but it is the best evidence I have, closest to my chosen time period, so I have tried this method first. We can always try something else.

To make gesso sottile, I fill a bucket with water and poured plaster in. It is always recommended that you add plaster to water, not water to plaster. When making a gravy or sauce, this produces lumps but for plaster it works well. I mixed it gently and continued to add plaster until it was not absorbed immediately. I covered the bucket with another tub to stop rain, cats, frogs and leaves getting in. Now, we wait.

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.