Tell me about torrified tops

[colins]

Originally Posted by Phil T

Hmm. Exactly what a Coke Oven does to coal to make coke (metallurgical)

Yep, same process but different temperatures. If you subject a piece of wood to steadily (and slowly) increasing temperature in an oxygen-free environment the volatiles start to boil off as you hit the low 200 s (Centigrade ). They then volatilise progressively according to their boiling points and this is not complete until up around 800 degrees C, which is where you get the highest fixed carbon yield but the lowest overall mass yield.

Col

[RalphH]

Originally Posted by Halcyon/Tinker

I’m not sure I agree that it’s stiffer. It’s been a long time since I had my first batch done, but I took careful measurements of dimensions, stiffness, and weight.

I did 20 tops, and IIRC, the only thing to change dramatically was the weight. Yes they shrank a tiny bit in all dimensions (most change to the width, losing 2-3mm per panel, which, since that change is fixed, is part of what makes torrefied tops less reactive to climate fluctuations), but the big change was the weight, which I believe was in the neighbourhood of about 15%.

When I first got my hummingbird out of the case I couldn't believe how light it was. Felt like it was made of balsa wood. 15% off just the top doesn't sound like a huge amount, but I can't think why else it would feel so light to me.

[varve]

After studying this for a few years, including the study referenced in an earlier post from the Berlin wood laboratory, I am convinced that a large part of the effect can be attributed to the fact that aged wood, whether by time or by torrefaction, is simply drier at a given relative humidity. The equilbrium moisture content (EMC) of the wood on your guitar top has a profound effect on stiffness (Young's modulus across and along the grain) and damping (internal friction of the acoustic vibration).

By the classic work of Obataya et al , spruce has a maximum stiffness, and a minimum damping, at 6-7% EMC. Above this level, stiffness decreases and damping skyrockets, with deleterious effect on sound. If you've ever noticed that your guitar sounds like it has socks in the soundhole during wet season, that is why.

Aged wood simply has a lower ability to absorb and desorb water, the practical effect of which will be that an aged top guitar will have a lower EMC than a non aged (or non torrefied) guitar. This means that the aged instrument will have both a greater dimensional stability with humidity changes, and will have whatever sonic advantages come from existing at that lower EMC. I had the great honor of presenting this information to the Fretboard Summit a few years ago, with Bill Collings in the front row-- arms folded across his chest and a skeptical look on his face. It was the first and last time I had the honor of meeting him, and I remember the moment with clarity. At the end of the talk, Bill unfolded his arms, nodded, and said two things: "Guitars sound best just before they crack", and "OLD guitars don't tend to crack when dry"

[printer2]

White spruce percent moisture content to RH



45% RH is about 7% moisture content, where we generally try to build at.