A technique developed in Sweden for curing wood to be used as decking is advancing the art of building acoustic guitars at Bourgeois Guitars in Lewiston. 

The decking was developed too late to compete successfully against synthetic products, but the process, called Torrefaction, is now finding use in electric guitars and violins. Bourgeois is the first to apply it to acoustic guitars, and just sold its first to singer, songwriter and rock guitarist John Hiatt.

Seeing the application of the treated Sitka spruce that makes up the face of the guitar is a fascinating lesson in the tuning of a stringed instrument — not the tension on the strings, but the myriad variables that influence its unique voice.

“The player experiences the result (of using the treated spruce) as a quicker, louder and fuller tone,” said Dana Bourgeois. “The swell of the harmonics takes place faster, and the notes can be played quicker.”

One of Bourgeois’ most famous guitars, a Slope D from 1995, was called “The Banjo Killer” because of its ability to compete with the banjo in a bluegrass band in terms of rapid notes and volume.

That guitar was submerged in a flood, and after a year of drying out, was disassembled and reconstructed by famed Japanese luthier Shin Ichikawa, who did the work without charge.

Bourgeois estimates that the number of permutations of every material and process that goes into a guitar is about a billion — two billion if you include left- and right-handed versions.

The face material, although only one factor, is probably the most important, although the back — made of rosewood, mahogany, maple or others — comes close, because it must complement the acoustic character of the face. 

“The back can provide more or less sympathetic harmonic coloration,” Bourgeois explained. “Maple has a drier sound. Mahogany and walnut provide warmth, and rosewood quite a bit of complexity.”

The company is experimenting with Pao rosewood, which is not an endangered species, and also with using Torrefaction on the wood of the back and sides.

Torrefacted wood, subjected to high temperatures in an oxygen-free environment, becomes both lighter and stiffer. In effect, it “freezes” the cellular structure of cellulose and lignans while driving off volatile materials and modifying the microscopic “check valves” that prevent sap from flowing downward in a living tree. The result is a longer sound chamber within the wood.

The effect is similar to what happens when wood is aged for 50 or 100 years, one of the factors that makes pre-war Martin guitars sometimes fetch $500,000. It also contributes to the vaunted tone of a Stradivarius.

Bourgeois has trademarked its new line of guitars, introduced this month at a trade fair in Nashville, as “Aged Tone.”   

When one picks up a shingle-sized piece of processed wood and compares it to the same size of standard Sitka spruce, it not only feels lighter and stiffer, it also rings like a bell when one taps the surface.

The pitch and harmonics depend on the location tapped, rather like a steel drum. It is the job of the luthier — the correct name for a master guitar maker — to generate as many clear “tap tones” as possible by shaping, shaving and bracing the face. 

The Aged Tone guitars also have a finish with the look, feel and sound of that on older instruments.

Bourgeois builds about 400 instruments a year, and will offer the Aged Tone in six different models. They range in price from about $5,000 to $20,000, depending primarily on the number of custom features. They’re worth every penny “if you want to be heard.”

Christopher Hyde is a writer and musician who lives in Pownal. He can be reached at:

[email protected]