Gilding the sella curulis

The sella curulis is a luxury folding chair, intended as a seat of authority. Therefore, I wanted to have the seat to have an appearance of importance, and impress by having gilded the eagle heads and claws. Gilding is 'common' during medieval times (but not cheap - for the ordinary man). Think of the gold leaf applied to illuminations in medieval manuscripts, or on altars and reliquaries in churches, on panels in palaces. The medieval method of gilding is described in the 15th century book 'Il libro dell'Arte' by the Italian painter Cennino d'Andrea Cennini, but also earlier wrote Theophilus (around 1120) notes on making and applying gold leaf to wood panels, as well as on how to gild metal, in his 'On divers Arts' . The gilding process is a laborious process that takes quite some time. The part that has to be gilded has to be prepared  first with several layers of gesso and bole, before gold leaf can be applied. As I had no experience in gilding, I first tried it out on the test head and claw feet. 

The test pieces. The eagle head is gilded with imitation gold, while the claw feet are gilded with real gold leaf. The test pieces also served to see whether a black and gold eagle head was better than a linseed oil and gold one.

The gilded Shrine of the Virgin, made ca. 1300 in Rhine valley in Germany, and now on display in  the Metropolitan Museum of Art, New York, USA. Closed, it is a statuette of the enthroned Virgin Mary nursing the Christ Child. Opened, the shrine shows a representation of the Trinity (the figure of Christ and the dove of the Holy Spirit are lost). The wings show painted scenes of the Nativity. The shrine is made from oak with linen covering, paint, gilding, and gesso. The size is 36.8 x 12.7 cm (closed) and 36.8 x 34.6 cm (open). The most right image shows a detail of the medieval gilding. The images are from the Metropolitan Museum of Art.

Preparing the work

I used the online course on water gilding (actually the water is water with hare glue) for the preparation of the wood. Basically, it consists of three steps: a glue layer, a gesso layer and a bole layer with some polishing in between. 

 The sella curulis after applying four layers of gesso.

In the first step, the wood has to be covered with a layer of hare glue in order to close all the pores. If this is not done, the gesso and as well as the bole will impregnate the wood and make it too wet inside, while drying to quick on the outside. However this only applies when using old-fashioned chalk-based gesso. Modern acrylic gesso does not need the glue layer, as it also closes the pores of the wood. I used the acrylic gesso, so I skipped the glue layer. Four layers of gesso need to be applied quickly one after another, when the previous layer has not completely dried, but has a half-dry matted glare. I was able to work two leg parts of the chair at the same time. When the second leg was painted with gesso, the first leg had dried enough to receive another layer of gesso. The application of the third and fourth layers is faster than the first ones. After drying the gesso can be sanded smoothly with 240 grid paper followed by polishing with a whetted cottoncloth. This way the gesso slurry will fill the small holes left during the application of the gesso layers. The sanding/polishing step is an impossible thing to do with all the feathers of the eagles head and was left out.

Left: the red bole clay (or fond) used. Middle and right: the sella curulis after three layers of bole.

The third step is applying the bole or fond. This is a clay-based layers and available in three (clay) colours: red, yellow and black. Red is the most commonly used and used by me as well. When cracks appear in the gold leave, the red will shine through giving it and 'antique looking' patina. Red bole can be bought ready to use, but needs some thinning with water (drops) to be able to apply it easily with a brush. Three to four layers of bole need to be applied, and each layer has to be completely dry  before a new layer can be added. I waited at least a day between two layers. Also here a very light sanding of bole layer 2 (600+ grid) can take place and polishing with 4/0 steel wool at the final layer. Also here, polishing feathers was not feasible. At this stage it is important (at least for water gilding) not to touch the bole layer with your fingers as they will leave fatty/oily fingerprints where the water/glue will not adhere to.

Gold leaf

 On the left side the larger sheet of imitation gold leaf, on the right the smaller 23.75 carat gold leaf.

 Theophilus describes the making of gold leaf as follows:

Take some Byzantine parchment, which is made from flax fiber, and rub it on both sides with the red pigment that is made by burning very finely ground and dried ocher. Then polish it very carefully with the tooth of a beaver, or a boar, until it becomes bright and the parchment sticks fast as a result of the friction. Then cut this parchment with scissors into square pieces, four fingers wide and equally long.

After this make a sort of pouch of the same size out of calf vellum and sew it together firmly. Make it large enough to be able to put a lot of pieces of the reddened parchment in it. After doing this take pure gold and thin it out with a hammer on a smooth anvil, very carefully, so as not to let any break occur in it. Then cut it into square pieces, two fingers in size. Then put a piece of the reddened parchment into the pouch and in the middle on top of it a piece of gold, then another piece of perchment and again a piece of gold, and continue doing so until the pouch is filled and there is always a piece of gold interleaved in the center. Then you should have a hammer cast from brass, narrow near the handle and broad at the face. Hammer the pouch with it on a large flat smooth stone, lightly, not heavily. After frequent inspection you will decide whether you want to make the gold completely thin or moderately thick. If the gold spreads too much as it is thinned and projects out of the pouch, cut it of with small light scissors made for this purpose alone.

This is the recipe for making gold leaf. And when you have thinned it out according to your liking, with the scissors cut as many pieces of it as you want, and with them ornament the halos around the heads of figures, stoles, hems of robes, etc. as you like.

Cennini just buys his gold leaf (just like me)

Let me tell you that for the gold which is laid on flats they ought not to get more than a hundred leaves out of a ducat, whereas they do get a hundred and forty-five; because the gold for the flat wants to be rather dull. if you want to be sure of the gold, when you buy it, get it from someone who is a good goldbeater; and examine the gold; and if you find it rippling and mat, like goat parchment, then consider it good. On moldings or foliage ornaments you will make out better with thinner gold; but for the delicate ornaments of the embellshment with mordants it ought to be very thin gold; and cobweb like.

Left: the booklets of imitation and real gold. Right: some pieces of gold leaf on transfer their transfer tissue.

One might think that gilding is expensive. This is only relatively so. As seen from the quote above a gold coin can produce almost 150 gold leaves - during medieval times. Gold leaf is beaten very, very thin, and nowadays gold leaf has a thinness of 0.0007 mm. Therefore, a booklet with 25 leaves of gold (8 by 8 cm) costs only around 45 Euro. I used 23.75 carat gold leaf (Rosenobel Double Gold extra strong) from the German gold-beater factory Norris for gilding. This gold leaf has been pressed on on a tissue, which enables easy transfer to the surface and makes it possible to cut the gold with a scissor into smaller parts. One entire book was enough to gild all the eagles heads, the claw feet and some test parts. 

The gilding process

In laying on [the gold] take glair, which is beaten out of the white of an egg without water, and with it lightly cover with a brush where the gold is to be laid. Whet the point of the handle of the brush in your mouth, touch a corner of the leaf that you have cut, ans so lift it up and apply it with the greatest speed. Then smooth it with the brush. At this moment you should guard against drafts and hold your breath because, if you breathe, you will lose the leaf and find it again only with difficulty. When the piece has been laid on and has dried, lay another piece over it in the same way, if you wish, and also a third, if necessary, so that you can polish it all the more brightly with tooth or a stone. if you wish, you can also lay this leaf in the same way on a wall or a ceiling panel [Theophilus].

The test pieces gilded using the water/hare glue mixture. You can see that the beak of the eagle head (with imitation gold) is better gilded than the claws and rings (with real gold leaf). There are many cracks and holes in the leaf gold of the claws and rings, seen by the patches of red bole.

I started, however, my tests with the much cheaper imitation gold or brass leaf. I did not use 'glair' or 'size' made out of egg white, but a hare-glue/water mixture. This easily glued the imitation gold, but the transfer of the real gold leaf did not go well. Therefore I looked for another adhesive and turned to the more modern (linseed) oil-based mixtion by LeFranc/Charbonnel. This went rather well, although a waiting time of 3 hours was necessary between the application of the glue and the gold leaf. 

The gilding equipment: the oil-mixtion, scissors for cutting the gold leaf, cotton gloves, 

an assortment of brushes and a gold duster to remove small bits of gold leaf.

As mentioned above, the bole must not be touched with the fingers. Therefore cotton gloves were worn during the actual gilding.  When the glue or mixtion was applied to the bole layer (and for the mixtion dried for 3 hours) the cut pieces of leave gold were pressed to the sticky glue with the fingertips in gloves and the transfer tissue removed. A soft brush was the used to flatten and smooth the gold leaf over the surface. The following gold leaf was applied overlapping the previous leaf. If cracks appeared in the gold leaf, a second layer of gold leaf was applied. Adding extra mixtion for the extra layers is unnecessary. A soft brush with a stronger tip was used to apply small pieces of gold leaf in corners, such as the eagles eyes).

First the mixtion has to be applied, this gives the wet look on the beak. After drying small squares of gold leaf are pressed with the glove on the mixtion, and smoothed with the brush. During the gilding many small bits of leave gold can be found on the table (and your clothes).

A detailed look of gold leaf application. The piece of gold with the transfer tissue can be seen at the corner of the beak. The right photo shows that another gold leaf needs to be added to cover the cracks between the upper and lower beak.

After applying gold leaf it is possible to polish the gold leaf. The gold glue must not be dry to do this. As my gold leaf was very thin and the oil-mixtion is very sticky, I did not try this. Polishing was done in the past with a tooth ('of any animals which feeds decently upon flesh' according to Cennini) or semi-precious stones like hematite or agate. Cennini mentions that winter is best for burnishing gold leaf, as the weather is damp and mild. If it is too dry, the gilded object must be kept in a damp place, like a celler at the foot of the wine casks. If the time after applying gold leaf is longer than a week or month, the object can be 'reset' for burnishing by applying a towel over the object and use another napkin, soaked in clear water and wrung out, laid on top of the first towel. The gold will then soon be ready for burnishing. 

The finished gilding of the sella curulis.

Painting the remainder of the eagles head

The golden beak, eyes and claws have a greater contrast and a more stunning effect when the remained of the head  (and feet) is black. I did not want to paint the complete chair as I like the 'pink' colour of the pear wood and the chip carvings will look better without paint. I made the black paint as previously described for the chess game boards: bone black powder with added linseed oil. Drying of the paint takes a long time, between one and two weeks time for the black paint, as Theophilus already remarked:

All the kinds of pigments can be ground with this same [linseed] oil and laid on woodwork, but only on things that can be dried in the sun, because, whenever you have laid on one pigment, you cannot lay a second over it until the first has dried out. this process is an excessively long and tedious one in the case of figures ..... On wood you should apply all pigments, whether ground with oil or with gum resin, three times. When the painting is finished and dried, carry the work into the sun and carefully coat it with the gluten varnish. When the heat makes it begin to flow, rub it lightly with your hand. Do this three times and leave it until it is thoroughly dried out.

 

Painting the head with linseed oil paint. First the edges with the gold leaf were painted, followed by the remaining part. The white gesso serves as a primer for the black paint. 

The finished feet and eagle heads of the sella curulis. Also the underside of the feet are painted.

The gilded and painted sella curulis. The remained of the wood will receive a linseed oil finish.

Sources:

• The craftsman's handbook 'Il Libro dell'Arte. Cennino d'Andrea Cennini, translated by D.V. Thompson. Dover publication. ISBN 978-0-486-20054-5.
• On divers arts. The foremost medieval treatise on painting, glassmaking and metalwork. Theophilus, translated from Latin by J.G. Hawthorne and C. Stanley Smith. Dover publication. ISBN 0-486-23784-2.
• The complete watergilding course by Pictureframes.co.uk on YouTube: Complete water gilding course