Thursday, January 29, 2015

Wood Sandwiches and Warped Doors

My Son was helping me out at the studio one day and he got a bit distracted. I know he said he was hungry but I wasn't paying attention.

Hence the salami and Swiss on rye sandwich.
Or rather amaranth and satinwood on pine.

I thought of this as I was thinking of this post.

The layering of wood into a "sandwich" adds some interesting and useful properties that can be utilized for fabrication and restoration.

Sandwiched Wood
Some History

Plywood has been with us arguably for centuries. I say arguably because  the references and discussion of purpose and reason vary.  The arguments go from stability of the groundwork to the conservation of materials  - and pun intended it falls somewhere in-between.
It has never been the material of choice for me - but it does have it's purposes. The oft quoted line is "If Chippendale had plywood he would have used it." He may have, but my gut feeling is that we would probably have fewer remaining examples of his craftsmanship.

By veneering your most precious materials on to a dimensionally stable groundwork you are conserving the valuable figured wood. Slicing the presentation timber into thin veneers allows you to cover a considerably larger amount of surface - as opposed to using the original board in solid.

Imagine a day on the end of this saw
This technique of cutting valuable timber into veneers was implemented regularly  in the mid 17th century for just this reason. The problem is that the process of cutting the veneers was done entirely by hand. A time consuming and very laborious process. Consequently, the fabrication of a plywood groundwork would have been much too labor intensive, therefore the resawn presentation veneers were typically laid upon a secondary wood of pine and sometimes mahogany.

The following earliest reference from an examination of an Egyptian coffin was done (in my opinion) entirely for stability. Wood expands and contracts with the addition and removal of moisture across its thickness. By layering thin sections of wood crosswise into an panel, the movement is minimized and counteracted by the opposing layers.

From Pharaonic Egypt
The oldest piece of plywood was found in a third dynasty coffin, made of six layers of wood each 4 mm thick and held together by wooden pegs. [11] Like modern plywood the grain of its layers was arranged crosswise to give it added strength. [12] From 1750 BCE onwards this plywood technique became widespread. The thickness of the layers was reduced to less than three millimeters and they were stuck together with a glue made from bone, sinew and cartilage applied hot. [13]

The Rush of the Machine Age

In 1797 Samuel Bentham applied for patents covering several machines to produce veneers. In his patent applications, he described the concept of laminating several layers of veneer with glue to form a thicker piece – the first description of what we now call plywood. Samuel Bentham was a British naval engineer with many shipbuilding inventions to his credit.

About fifty years later Immanuel Nobel, father of Alfred Nobel, realized that several thinner layers of wood bonded together would be stronger than one single thick layer of wood.

 Plywood was introduced into the United States in 1865 and industrial production started shortly after. In 1928, the first standard-sized 4 ft by 8 ft plywood sheets were introduced in the United States for use as a general building material.  From Wikipedia

With the advent of this invention it was now possible to manufacture engineered panels with increased strength and dimensional stability. A stable groundwork for veneer that was resistant to warp and shrinkage splits.

Which leads me to the subject at hand... warped cabinet doors.


Warped doors on cabinets are a fairly common occurrence. Often they can be coaxed into an acceptable position by adjusting the hinges. This doesn't remove the warp but only balances the panel and frame in the opening.

In the case of our current patient a more invasive procedure was required. (Using what we've delved into previously plywood seems an option.) The stiles were warped and twisted enough to pull the door almost 2" out of plane when closed. Even the original heavy duty compression latch wasn't enough to keep it closed.

Taking apart the wayward door was in this instance almost half the battle. Being a newer reproduction piece the joints were glued with synthetic modern adhesive which proved to be rather resistant to dis-assembly.

Once the door was apart the stiles were bandsawed through their entire length to remove the tension in the wood caused by the warp. Before proceeding to the next step the saw kerf was cleaned up with a hand plane.

A new piece of wood, the thickness of the material removed by resawing was then glued between the 2 pieces of the stile and clamped on a flat surface to eliminate the distortion. A wood "sandwich", or in essence plywood. By gluing up the pieces with a core of quarter sawn kiln dried birch, the tension was distributed between the three pieces and the glue joint.

After removing the flattened piece from the clamps, the excess material was trimmed to blend with the original. The joinery was cleaned and refitted to correct for any remaining distortion and the door was re-assembled.

Stile in clamps
The "Sandwich" prior to trimming

The Completed Project

Closeup of spline

Additional Photos Here