Monday, 11 August 2014

The leather seat of the sella curulis

Although adding a leather seat to the sella curulis seemed an easy task at first, it was rather more complicated. Not in the first place because I made several mistakes which had to be redone before the leather seating finally could be attached. The seating problem started with the question of how these seats originally were attached. The images of the extant pieces were of not much help as the attachment of the leather is not seen when they are shown at their best angle. The faldisorium of the Nonnberg convent had the leather between two wooden rails. Was the leather clamped between these rails? Was the leather folded at the end, so that it could not retract between the rails? Or was the leather nailed to the lower rail and served the upper rail only for decorative purposes?  One of the two folding chairs of the Museum für Angewandte Kunst in Vienna showed some nails. But how many were used and how were they attached? There were simply too much unknown factors to start straight away. 

The sella curulis from the Stift Nonnberg, Salzburg, Austria. The leather goes between a decorative upper rail and a smaller lower rail. 

Seating detail of the 15th century folding chair from Stift Admont, Austria, now in the MAK, Vienna. On the right side you see that the leather is folded over the rail and then nailed to it (red arrows) with at least seven nails. The leather seating itself is folded at the edge and sewed (green arrow), providing a more comfortable seating.


The solution was just to ask. The curator responsible of the furniture and wooden objects of the MAK, Herr Sebastian Hackenschmidt, was kind enough to answer my questions and he send me some detailed photos of the leather attachment. The leather of the seating consisted of two pieces, sewn on top of each other and nailed to the connecting rails. The the method of nailing was clearly visible on the photos. The points of the nails were bent and then hammered back into the wood. Nine nails were used to attach the leather to each rail. Herr Hackenschmidt also noted that the (bottom) rails containing the arms of abbot Johann von Trautmannsdorff (who headed the cloister from 1466 to 1481) to the two legs were an addition of the 15th century. Possibly before that time a connecting rail used to exist at the X-point, which was weakened and replaced by new bottom rails and some iron reinforcements inside at the X.

Inside of the seating rail of the 13th century sella curulis from the stift Admont, Austria, now in the MAK, Vienna.  Eight nails can be counted, the ninth is hidden behind the leg. Note that the leather of the seating is double and used to be sewn together (the sewing holes can still be seen). You can also see that the inside of the joint for the legs is reinforced with an iron rail. Photo kindly supplied by S. Hackenschmidt, copyright MAK, Vienna.

The outside of the same seating rail. Seven of the nine nails can be seen. At the side the double leather of the seating is folded over. Photo kindly supplied by S. Hackenschmidt, copyright MAK, Vienna.

The leather

I am lucky that my neighbour (Gelderse Roos) makes historic shoes (and costumes), which means that it was rather easy to acquire leather for the seating. They did have a piece of old leather that had been used for seating, approximately 2 mm thick. First, the leather was cut to width. The wooden rails have a width of 32 cm, so the width of the leather including two 1.5 cm folds was set to 35 cm. As I wanted the leather to be coloured black, to fit with the back and golden eagle heads, my first task was to clean it of any oily substances of its previous chair life with acetone.

The piece of old seating leather I started with.

 
The shiny leather look is gone after applying an acetone treatment. 
The rag has become brownish from the dirt of the leather.

Then the leather was cut to width. The wooden rails had a width of 32 cm, so the width of the leather including two 1.5 cm folds was 35 cm. The edges of the leather were pared (bevelled) to make the folds thinner, using a leather skiving machine. During medieval times this was done with a very sharp skiving knife. The technique how to pare leather manually is shown in this youtube film.

My neighbour at work with the skiving machine. It has a razor-sharp knife set at a fixed bevel and a rotating wheel that presses and moves the leather along the knife. On the right photo you can see the line of leather that has been cut away.

Two more images of the skiving machine and at the bottom the results: the pared leather for the seating with a chamfered edge and the thin lines of leather that were removed.

With the leather pared to a bevel, it had to be coloured black. For a thorough black colour my neighbour recommended the alcohol based black leather paint of La Industrial. The paint was lavishly applied several times to both sides of the leather. The whole working place stank for a week of the fumes from the paint, but the result was a nice black coloured leather.

There is also a more 'authentic medieval' way to colour your leather black. In fact, if I had known it (and not just as I type this text) I would have tried it, as the recipe seems quite easy. To quote Toki Medieval:
"To dye leather black, you need acid, iron, and a tannin, just as you do when creating black ink. The leather has already got  tannin in it from the tanning process and simply dyeing in an iron pot would provide the iron, just as dyeing weld in an iron pot produces green instead of yellow.
The purpose of the acid is effectively to rust the iron, however, so rather than damage a pot I soaked iron wire in a vinegar solution in a glass dish, and then submerged the leather in it. The impact on the leather was rapid: within an hour it was already turning a blue-grey. It dyed best when floating just below the surface of the water, presumably because it was more exposed to the air. Where layers of leather overlapped, the lower layer didn’t dye as well.
I took it out after two weeks as if you leave it in too long, the acid will eat away at the leather, causing it to decay. At this point it was a deep blue-black which, after oiling, came up as the intense black shown below. Unlike with a modern dye, iron and vinegar turned the leather black right through, rather than just the surface.
"


 The leather painting process. The paint was soaked through the leather. Right: the black paint bottle.

With the black leather dry, the next step was to fold the edges and sew them. Actually, it is better to fix the paint first with a satin finishing mixture (explained later), as it will keep your hands clean, instead of black from the dried remnants of paint. Also it will keeps your sewing thread clean, instead of becoming greyish. First, we applied some leather glue (to make the sewing process easier), the leather was folded and hammered flat with a shoemakers hammer. Then the edges were sewn using a Rafflenbeul shoe sole stitching machine (a renowned high quality leather-working machine). It uses waxed linen thread, which is slightly pre-warmed by the machine for smooth stitching.


 Applying glue and folding the edges of the leather.

My neighbour is flattening the fold with a shoe-hammer on a wooden block.



My neighbour at work sewing the leather seating. Although the Rafflenbeul can be used fully automatically, it is more precisely controlled when moved by a hand wheel.


Then, I discovered my first mistake. The fold also added a few mm to the width of the seating, so my final width was not 32 cm, but 33 cm! This was too wide to fit on the wooden rails, and the stitching of one side had to be removed, the width adjusted, bevelled again, folded again and sewn again. The result was not as nice as the first go, but still acceptable.

The leather was then finished with a water-resistant coat from Eco-Flo. This is a modern acrylic finish that protects the leather against stains and provides a nice satin finish. It also protect the person seating on it from any black stains from the leather. When applied earlier it would have given a clean stitch. The coat was applied with a wet sponge, and after drying 14 hours, a second coat was applied, and after drying the leather was buffed with a soft cloth. Both the front and back-side of the leather received the finish. I also tested other finishing coats, such as leather oil and leather wax, but this did not give satisfying results with the painted leather.

 The coat had to be applied with a wetted sponge in long even strokes.

Finally, I thought that a gold thread was perhaps better suited than a linen thread. As a result, I painted the visible parts of the stitched linen thread with gold paint. Ideally, I should have used gold thread for (hand) sewing.

The leather with white stitching (left) and golden stitching (right). Actually, this is the third leather seating.

Here the difference between the plain sewing thread and the gold painted one is shown.

The nails 

As shown on the original faldistorium from the MAK in Vienna, the leather is nailed to the rail and the nails are hammered back into the wood. The MAK folding chair has nine nails per side. I tested the space between the nails on a piece of leather to find an aesthetically pleasing spacing and also ended up with nine nails per side... 



The three nails on the left are too close together, the space between the two right nails is more pleasing to the eye.

The nails have to be thin enough in order to be able to bend back into the wood. Otherwise they would either break or demolish the wood. My (machine)-forged 4 cm long nails were too thick (3 mm square) to bend and had to be made thinner. I created a wooden jig to be able to file the nails to half their thickness - at the part that was to be bended. The jig ensured that I did not had to hold the nails by hand, that they were correctly positioned for filing, that two nails could be filed at the same time, and that the nail length to be filed could be adjusted. Quite much for a scrap piece of wood with two holes in it. 


A hole slightly smaller that the nail is drilled through the wood (left photo, most left hole), and the nail is hammered through it and pulled out. A tight fitting square hole is left (left photo, middle holes) where the nails can be pushed through at the appropriate length (right photo).

The jig is set into a vice. Two nails can be filed at the same time.

Some filed unpolished nails.

After filing, the nail points were resharpened (also with the file). Next, the filed surface of the nails was sanded with 120, 180 and 400 grid paper. The nail heads also needed to be gilded. Gilding nail heads (of gilding eagle heads) has the advantage that you can push you nails on the gold leaf, instead of otherwise. This makes gilding an easy process. I used Indian 24 carat gold leaf, which is sold (relatively) cheap in booklets of 100 loose leaves on internet. The leaves were cut in four and placed on a foam mat. The nail head was glued with super-glue and then pressed onto the gold leaf. The edges of the gold leaf were folded over with help of a brush. The process was repeated, so each nail head had two layers of gold. Reason for me to use super-glue and two gold layers was that I expected quite some force on the heads during hammering of the nails.

The nail head gilding process: the foam layer allows to press the gold leaf against the nail head, without damaging the ultra thin gold leaf.

 A batch of gilded nails.


Nail heads can also be 'gilded' by pressing a glued head into bronze powder.   


Construction

At this point I tested the length of the leather for the seating, to be sure nothing went wrong with the size. But then the inexplicable happened, I cut it the wrong size and the length was about 9 cm too short. I had to redo the whole leather seating. This time new saddle leather was used, which did not need the acetone treatment. (Alas, we did a miscalculation here as well, so we had to cut a third piece of leather. Luckily, we discovered this before we did anything else with it). Also the finishing coat was applied earlier, resulting in a clean stitching. Another improvement was to pre-fold the edge when the leather was wet with the black paint. Leather, especially thick and stiff saddle leather, is easier pliable when wet. With the edges folded the seating had a width of 32.5 cm.

Testing of the length of the leather seating on the sella curulis with an expendable piece of leather. Some spruce rails were used and the leather was stapled onto the rail. You can see the extra leather hanging over the right rail.

The leather had to be folded over the complete rail, such that the edge was hidden from view (unlike the two sella curulis in the MAK in Vienna). Also here I tested the attachment first with some spare nails and a scrap piece of leather.


The back and front of the nailing test.

For the attachment first 3 mm holes were drilled in the wood of the rail at the places where the nails would be placed. A modern nail was used to transfer the location of the holes onto the leather. The dips left in the leather by the modern nails were punched through with a leather punch or a revolver punch pliers. The wooden rails were also finished with linseed oil, except for the tenons, before the leather was attached.

Transferring the position of the holes in the rail onto the leather using modern nails.
 Two rows of punched holes for the gilded nails.
The two rails on top of the leather. The pieces of leather on the left and right are to be folded over the rail.


Then the gilded nail was pushed through the leather, the hole in the rail and again through the leather. The nail-head was pressed flat against the leather with help of another simple jig. This consisted of a wooden block with a hole drilled in it. The hole was placed over the nail point and hammering on the wooden block pressed the leather and the nail flat. For protection of the gilded nail head, a piece of scrap leather was placed beneath it.

  Left: Another simple jig: a wooden block with a hole drilled in it. Right: Here the block with the hole is set over a protruding gilded nail. A piece of scrap leather protects the gilded nail head.

The protruding nail point was bended at 90 degrees with help of pliers, before being hammered down back into the wood. Now only the sella curulis parts have to be put together and the chair will be finished. But that will be a topic of a next blog post.

Left: the bent nail. Right: four of the nails hammered back into the wood.


The finished seating, with all 18 gilded nails.


The inside (left) and the outside (right) of the seating rail, with 8 gilded nails.

 
The finished seating of the sella curulis. You can see that the saddle leather is stiffer than the leather of the 'first seating attempt', and is tilted slightly upward. This will improve when the chair is in use.  

3 comments:

  1. The sella curulis looks great Marijn, congratulations! The black and gold does work very well with the colours on the woodwork. Thanks for giving both the successes and difficulties that you discovered while making the seat. It's just as interesting to see what didn't work for you as what did and why some things didn't work; very good information to have available for many of us other makers I'm sure. Good luck with the next project, I look forward to reading about it!

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  2. Can you tell me the approximate dimension for this sella curulis?

    Has the plan show 2 pages later (with the 5 cm scale) seems kind of gargantuan!

    (nice work!)

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  3. I have been your silent reader for a while, and now I think you should know how valuable and helpful the information and tips you have shared with us. Thank you for sharing. Would love to see your updates again and maybe we can share ideas and collaborate with each other in the future.

    Elcometer 456

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