Sunday, October 10, 2010

The Assembly Board

Marquetry woodworkers and students who do not have a chevalet always ask me, "Would I learn anything useful if I took your class and did not own a chevalet?' It's an excellent and logical question.

First of all, most people who have tried their hand at marquetry have used a knife, fretsaw or jigsaw. They either do not have the room or money to invest in a fancy cutting tool which is foreign to them. Old familiar habits are hard to change.

However, there is an important and fundamental part of the French process for making marquetry which works in all cases, independent of the method used for cutting out the pieces. The French name for this is "cale tendue" which translates directly as "paper stretched over a board." Obviously this name just doesn't roll easily off the tongue. So, in order to teach students and explain to other workers what I use, I have adopted the name "assembly board."

Most people who make marquetry pictures work from the face of the design. If they use the bevel method or window method or any other method, they normally build the picture looking at the front and use glue or tape to hold it together. One of the drawbacks of this is that, if the design is very complicated, you need many layers of veneer tape to hold it properly. These layers are not even over the entire surface, so that when you press it the pressure hits the top of the tape on the high spots, and the veneer is not held in place. This can cause wrinkles, as the veneer moves before the glue sets.

There is another problem with working only from the face. It is impossible to add a mastic, which would be used to fill the gaps left by the kerf of the saw, from the front without damaging the veneers. Mastic is normally made of dilute hot glue and fine sawdust, and when it dries it is like cement. I compare it to tile grout, since it actually forms a structural bond between the elements, and also makes the surface flat under the finish.

The French solved these problems centuries ago by creating an assembly board. They use a Kraft paper, which is common all over Europe. This paper is made the old fashioned way, so that its thickness is made up of layers of paper fibres. All paper made this way can be split down the center of the thickness into two surfaces. Unfortunately, book collectors have used this trick to remove historic prints from books and split the single page into two pictures. The front picture is split away from the picture on the back of the same page, so that two pictures can be sold where a single page existed before. Many great books have been destroyed by this trick.

The Kraft paper also has two different surface treatments. One side is glossy and resists moisture and the back surface is matt which absorbs moisture. Normal American butcher paper is not like this. American paper is finished the same on both surfaces, and cannot be used for making an assembly board. I have not found an American paper supplier of European Kraft paper, so I have to import large rolls of it at some expense for my work. I make it available to students at a modest price, so they can continue the work here in this country.

To make an assembly board you do the following: Start with a board (solid wood, plywood or MDF) larger than the final picture. Cut a piece of Kraft paper large enough to cover both sides. Lay the paper glossy side up on a waterproof table and wet the paper with a sponge. Turn the wet paper over so that the matt surface is up, and make sure this surface is dry. Wait a few minutes and the paper will expand, creating wrinkles. Stretch the paper again so that there are no wrinkles. Put hot hide glue on all the edges of the board and lay the board down on the dry surface of the paper, so that it is only on half the page. Working quickly, fold up the paper around the three edges of the board and stick it to the glue. Take a razor blade and cut around the three edges, with the blade in the center of the edge. Remove the scrap of paper which is cut away and then fold the rest of the paper over the top of the board and stick it to the glued edge like before. Again take the razor blade and cut around the three sides along the center of the edge. At this point you have the wet surface of the paper showing on the outside of the board and the paper glued around the entire edge. Add veneer tape to the edge as a final attachment to hold the paper in place. Place the assembly board in a warm place to dry.

As the paper dries it shrinks. This produces a flat surface with the paper pulled tight and the glossy surface on top. The board has two sides which can be used, making it practical for two pictures, one on each side. Often we prepare boards in advance so that they are handy when we need them.

To build a marquetry picture we always take a prepared assembly board and put hot glue on the paper. Then we put the veneer face down in the glue and press it on the paper. All the elements of the picture are placed face down into the glue, working quickly. If the picture takes longer to put together, and the glue starts to gel, we add fresh hot glue with each additional piece. It is also possible to reheat the elements with an iron and remove or adjust them as required. When the picture is fully assembled, it is pressed flat onto the paper for a few hours using a piece of plexiglass.

By this method all the elements of the picture are pressed fully forward so that the picture face is perfectly flat. If there is any difference in the thickness of the materials they can be adjusted by sanding or scraping away from the back surface, since all pieces are glued in place on the paper face down.

Finally, a mastic of glue and sawdust can be applied to the back surface of the veneer, filling all gaps. It doesn't matter that this mastic creates a mess on the veneers, since the back surface is also the glue surface, and hot glue bonds to hot glue. The last step is to lightly sand away any bumps or roughness created by the mastic, once it dries. Now the picture is ready.

Cut away the paper around the picture and you have a marquetry panel that you can use directly, store for future use or send to another woodworker to use in his work. The picture is glued down on the final project with the Kraft paper on the face. After the glue sets, the worker applies water to the paper (which is now the matt surface showing) and the paper begins to dissolve. During several minutes of water application and scraping the paper and any glue residue is easily removed from the surface of the picture showing the beautiful marquetry for the first time. By this technique it is not necessary to remove any veneer at all and the surface is nice and flat, with mastic filling all the gaps, ready for finish.

I know you will have questions about this. Trust me, it is the greatest thing since sliced veneer.

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