Making a sedia tenaglia part 2 steam bending

Did steam bending of furniture exist in medieval times? That is a difficult question to answer. It is very likely that it did exist. At least the technique was used in the medieval body-building industry. Wooden planks for the hull had to be bend before they were clinked with nails to the frame and the overlapping planks, and this was much easier to do with the help of steam.  Archaeological finds of lapstrake built boats also confirm that steaming of the wooden planks was used in boat building.

Noah building the ark. A favourite medieval boat-building scene. You can see that the boat is nailed and lapstrake built, but no steam-bending is being done here. Histoire Ancienne jusquà César. Manuscript Add. 15268 folio 7v. British Library, London, UK.

 

A reconstructed steam-bending scene from the Vasa Museet (Stockholm, Sweden). 

The Vasa was built in 1626-1627.

Another indication that (steam)bending was used in a more medieval house-hold context are the splint boxes. These boxes were already known in early medieval times (e.g. grave finds from Oberflacht in Germany, 6-7th century) and are still in use. Thin pieces (a few mm thick) of wood were bent with help of water, heat and steam to make oval or round forms. More on medieval splint boxes can be found on the blog of the Wienische Hantwërcliute 1350.

Spanschachtel box from the Augustinerchorfrauenstift Steterburg, 1st half of 14th century. Height 11 cm, length 29 cm, width 20 cm . Now in the Herzog Anton Ulrichs-Museum, Braunsweig, Germany. 

Medieval splint box with its contents: a mandrake in silk wrappings. The box was found under the choir stalls in Kloster Wienhausen, Germany. Image scanned from the booklet Der Fund vom Nonnenchor by Horst Appuhn.

Whether medieval furniture also was made from steam-bend wood is not clear. Some photos of extant pieces suggest that they were bend, but no information is given if this was really the case. Nor have I been able to check if the wood grain of these chairs follows the flow of the bend rails. Anyway, I have used the steambending technique for my sedia tenaglia.

Making the bending jig

To steam-bend wood a bending jig is necessary. The wood is pressed against the jig and takes the shape of the jig. My bending jig was made of two pieces of 2 cm mdf board, which was glued together to get a 4 cm thick board, i.e. the same width as the wooden rails. I roughly used my plan of sedia tenaglia to transfer the curves to the mdf board. Then, I hammered some nails along this first sketch of the curve and placed a thin strip of wood against it. This thin strip bends easily and forms a natural flowing curve, while being kept in place by the nails. The final curve of the wooden strip was now transferred on the mdf board with help of a pencil. A band-saw was used to saw the curve, and a belt sander to smoothen it. 

 

Glueing the two pieces of mdf to get a 4 cm thick board.

 

Using nails and a thin strip of wood (triplex)  to crate a natural flowing curve. The lines of the initial measurements can be seen on the mdf.

As all the wooden legs need to have the same curve, some additional part were needed for the bending jig. An end-stop was needed to ensure that the curve started at the same point for each leg. A flattening clamp was needed to ensure that no bending occurred below that point (i.e. the part of the X needs to be straight, only the backrest needs bending). The flattening clamp also made it possible to quickly slide the steamed leg into the jig.

 

The end stop and the flattening clamp of the bending jig.

 

Here you can see the jig with an unsteamed leg inside it. The arrows show the part that will be kept straight

 and the part that will form the bended backrest. 

The steambox

The steambox - used to steam the wooden rails - is modern made from laminated plywood. The inner edges are sealed with silicon kit. The ground surface has a slight angle towards the middle, so the that condensed steam will flow back into the kettle. In the middle of the box is a hole with a PVC pipe connected to (the lid of) a glue box beneath which is filled with water. The glue box is placed on an electric heater. Another version of a steambox can be found on the blog of Johann von Katzenelnbogen, who makes traditional and early medieval handcrafted furniture.

Schematic overview of the steambox.

The plywood steambox with the gluepot and the electric heater in the middle. 

On top of the steambox is a lid with two handles.

Two legs of my sedia tenaglia could be placed together in the steam box. They rest on wooden blocks so they do not get unevenly wet by the condensed water whichis flowing back to the PVC pipe/glue-pot.

Inside the steam box. Two legs are waiting on small wooden blocks. 

You can see the hole for the steam and the returning condensed water.

The steam-bending process

The wooden legs stayed in the steambox between half an hour to one hour. One at a time was taken out of the steambox with fingered oven mitts (initially the wood and the steam are hot) and quickly put into the jig. Then clamps were applied, starting from the middle and following the bend towards the end. It is necessary to have wooden blocks between the clamps and the steamed wood to spread the pressure. If you do not do this, the clamps will produce dents in the wood, which are difficult to remove. (With help of a clothes iron with steam you can try to inflate the dent again, but you must take care not to burn the wood instead). After half an hour, when the wood had cooled down, the clamps were removed and the bended rail was transferred to a spare jig, were it was clamped again for some hours. Meanwhile, the first jig could be used to bend the second rail.

The first leg I tried to bend was about one cm thicker than the ones finally I used for the chair. The thicker leg could not be bend properly and veered back to a wrong angle. Perhaps this was due to a too short time in the steambox, but I decided to reduce the thickness of the rails. The thinner rails all bended perfectly well (except one that splintered), finally resulting in a chair having a lighter weight.

 

It is necessary to use wooden blocks between the iron clamps and the steamed leg of the chair. Steamed wood is soft and the pressure of the clamp will leave an ugly dent in the leg if no block is used to spread the pressure evenly.

At the straight end, hardly any iron camps are used.

Clamping starts at the straight end and then follows to the end of the curve.

 Sometimes the pressure of the previous clamps has to be increased when the bending proceeds.

Not all bending goes well. This leg cracked and split during the bending process. 

I had to make another one. Luckily it happened only once.

I will conclude the making of the sedia tenaglia in a next post.

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A late 15th century folding chair the sedia tenaglia

The sedia tenaglia is a folding chair with a backrest that first appeared in the late 15th century and persisted throughout the 16th century. Its main area of production was Italy, but these chairs were also found in the Alpine region (Switzerland, France and Austria). It is curious that these x-chairs appeared so late on the medieval scene, as the basic construction is not very different from the Savonarola chair or the late 14th century x-chair, but their seating is far more comfortable (at least my replica is). The main difference with the other x-chairs is that the X is projected at the side of the chair, instead at the front. This makes it possible to extent the legs of the X on one side and to create a (comfortable) backrest.

These two chairs  are from the Museo Storico della Caccia e del Territorio in Cerreto Guidi (Florence) Italy. They were made in Tuscany, date from the 15th-16th century and are made from walnut. The height is 78 cm, the width 37 cm and a depth of 26.5 cm. The seating is quite high, I guess around 50 cm (our seating height is around 43 cm). Both chairs have 7 back legs and a decorated back with circles.

Quite some pieces of the sedia tenaglia have survived and can be found in museums around the world or even be bought at antique houses. The surviving chairs are mostly made from walnut or beech. Beech has the benefit that it is easily bended by steam. The number of legs (for the backrest; the other legs usually number one less) can be an odd or even number and varies between 5 and 7. The wooden plate for the back is usually decorated with circular patterns with a cross in the middle. The following photos give a nice impression of the variety and sizes of the folding chair. The sources of the images are indicated; else they come from museum websites or other places on internet. One of the next posts will concern my construction of a sedia tenaglia.

This late 15th, or early 16th century chair is for sale at an antique dealer for 4800 dollar (at the time this post was written). The following photos show the specific details of the chair. The chair is 80 cm high, 43 cm wide and also 43 cm deep. The wood used for construction is beech.

You can see the wooden hinge dowels between the legs and seating rods.

Not only the back plate is decorated, also the legs and seating have some carved decoration.

 

The foot rail at the back is fastened with two dowels in the middle (left), 

while the foot rail at the front has two dowels at the ends (right). 

 

The circular decoration of the back plate. The backplate is fastened to the legs with two dowels at either ends.

The back side of the chair. The side of the chair; strangely the front legs look larger than the back legs.

The following six photos of a sedia tenaglia were made by Gary Halstead in a museum in Strasbourg, France. The chair originates from Switzerland or northern Italy and dates from the 15th century. The chair is made of beech and has seven legs. The photos were found on the greydragon website. 

 The seating of this chair is not very deep.

These two photos of the backplate show that it was fastened by two dowels at the ends. You can also clearly see that the decoration was made by hitting special punches into the wood.

The outer seating rails are larger and have a sawn out decoration. You can see that the legs are still a bit rough and bear the saw marks.

A sedia tenaglia from Tirol, dating from the 16th-17th century. Kunstgewerbemuseum, Berlin, Germany. 

Height 90 cm, width 46 cm. The chair has six legs. Image from the book Oude Meubels by Sigrid Muller-Christensen.

A simple French sedia tenaglia from the late 15th - early 16th century. Château de la Rochelambert, St. Paulien, France. As the chair is quite fragmented, some construction details, like the front dowel, can be clearly seen. The chair has six front legs as well as six back legs.Image scanned from the book Mobilier moyen-aye - renaissance by Monica Burckhardt.

This sedia tenaglia can be seen in the Museum fur Angewandte Kunst in Frankfurt, Germany. The chair has likely 5 legs.

Behind the table a sedia savonarola can be seen.

This is the only contemporary image I could find of a sedia tenaglia. It appears in the clothing book Trachtenbuch of Matthaus Schwartz of Augsburg, in an image of 1538 when he is 41 years old. The chair is small and only has 4 legs at each side.

These two chairs (3 photos) are from the Palazzo Madama in Turin, Italy and date from the 15th century. 

They have similar decorations as the one of the auction, circles with a cross. The 7 legs of these chairs are bend.

Left: A sedia tenaglia made in Italy from beechwood. It has a height of 80 cm, a depth of 48.8 cm and a width of 51.3 cm. The seating height is 47 cm. The chair has 5 legs and a decorated back plate. The chair is now on display at the Philadelphia Museum of Art, Philadelphia, PA, USA. Right: A sedia tenaglia with seven legs dating from the early 16th century and made from beech. 86.5 cm height, 44 cm width and 27.5 cm deep. Castello Sforzesco, Milan, Italy. Image from the book Mobel Europas II-Renaissance - Manierismus by G. Windisch-Graetz.

Left: A sedia tenaglia made in Italy from beechwood. It has a height of 80 cm, a depth of 48.8 cm and a width of 51.3 cm. The seating height is 47 cm (yes, similar to the chair above). The chair has 6 legs and decorated legs and back plate. The chair is also from the Philadelphia Museum of Art, Philadelphia, PA, USA. Right: This sedia tenaglia from the Morando Bolognini Museum, SantAngelo Lodigiano, Italy dates from 1490-1510. The sizes of this chair are: height 84 cm, width 37 cm and depth 29 cm. This chair has only 5 legs and misses the two outer seating rails.

A 16th century sedia tenaglia  from Burg Kreuzenstern near Vienna, Austria in seating and folded position. The chair has 6 legs and is made from beech, 80 cm high, 45 cm wide and 43 cm deep. Image from the book Mobel Europas I -Romanik - Gotik by G. Windisch-Graetz.

Left: Decorated folding chair with 6 legs from Tuscany. There is a bend  of the legs at the seating level. Collection Nella Longari, Milan, Italy. Right: Chair originating from Saluzzo, end of the 15th century. Italian private collection. Both images scanned from the book Il quattrocento - Mobili - Arti decorative - Costume .

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Making a sedia tenaglia part 1

We use our medieval replica furniture not only during re-enactment (such as at castle Hernen), but also in everyday live. Most folding chairs as well as my strycsitten are arranged along the dining table. As we were already planning to move to a new and larger house, there was a need for a larger table and extra chairs as well.  I did want to make a different medieval chair and try to steam bend some wood as well, so my choice was to make a sedia tenaglia.

As photos and information on internet only tell you the height and width of a chair, and not the thickness of the wood used, I looked at an example that was easily available: the replica sedia tenaglia at castle Loevestein. Another great source which I used was the construction plan of Charles Oakley of a 16th century German folding chair, which is actually an Italian style sedia tenaglia. The thickness and width of the legs of the chair in Castle Loevestein were 2.5 and 4.5 cm, respectively. That of the chair of Charles Oakley was 2.54 and 5.08 cm (1 and 2 inch), respectively. My thoughts were going in the same direction 2.5 and 4 cm, respectively. The thickness changed during construction of the chair to 2.0 cm for reasons explained later. This is still robust enough to sit on, and saves weight.

  

The replica sedia tenaglia at Castle Loevestein, Poederooyen, the Netherlands. This chair does not have a bend backrest. As such it can be nearly flat folded. On the other hand if you lean to much backwards, the chair (with you on it) will tumble backwards. This does not happen if the back of the chair is slightly bend.

Measurements of the folding chair at Castle Loevestein; the width of the chair is approximately 50 cm. The black dots indicate the placement of the pins for the mortise and tenon joints.

As I wanted to have the same seating height for the sedia tenaglia as my x-folding chairs, 44 cm, this measurement was also fixed. I also liked to have a deeper seating plateau than most sedia tenaglia, and chose a depth of 35 cm. Using these sizes, the folding X part of the chair could be drawn. Only the height of the backrest and the curvature of the back were left to be determined. The optimum height for the back of the chair depends on the people for whom it will be made (mainly me and Anne). The optimum curvature depends on the stability of the chair when leaning backwards (i.e. the chair does not tumble backwards when you lean on it), the ergonomics of your back while seating, and the wish to have the chair as flat as possible when folded. I think I succeeded very well in finding the optimum measurements for my sedia tenaglia.

Most of the remaining medieval sedia tenaglia are made from either beech or walnut. I chose oak to make my chair, as the table and all my other chairs were made of oak. Oak is a more difficult material to work with, and tougher to bend. I started to cut and plane the oak to the appropriate thickness and width. The next step was to bend the back, which I will cover in the next post. 

Meanwhile, also the caps for the dowels had to be made. I used tenon cutters in the drill press to make the caps. The trick is to have the piece of oak thicker than the tenon cutter can cut deep. This way, the caps are still attached to the piece of oak which makes is easy to centre the drill bit to make the hole in the centre of the cap to attach it to the dowel. This hole is cut just deep enough to hold the dowel. Then the caps are cut loose with a (Japanese) saw, and if you planned it well, the second round of caps is already waiting for you beneath the previous ones. You just have to drill the centre holes and saw these loose as well. Rounding of the caps is done similarly as with the Savoranola chair: using a belt sander or a rasp.

Seven times two caps are waiting in this piece of oak. 

The tenon cutter has done its job and the first seven caps have their centre hole drilled.

The seven caps are cut from the oak piece along this line (arrow) with a Japanese saw (kataba).

The first seven caps are cut loose; the second row of caps is already present due to the deep drilling of the tenon cutter.

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