Implicit in that data point you cite is, perhaps, a recommendation that 30-40% RH is optimal?
Not really, Fazioli were after a patent so they did comparative test under gwing's "desert" conditions to prove their point. The patent describes the test as follows, slightly abridged:
It will also easily be understood that, with such configuration of the Sounding board, the danger of the occurrence of cracks is completely canceled or in any case it is dramatically reduced. In fact, also in environments characterized by a low humidity, the layer of adhesive between the two panels will stiffen the structure of the sounding board itself, so as to prevent that the planks of each panel separate one from another, so producing the cracks cited before.
It will then be noted that .. the adhesive between the two panels will contribute at any crack to maintain mutually bound the edges of the crack so preventing the occurrence of a through opening in both panels.
To that purpose, experimental tests have been performed in which the behavior of two samples of a sounding board have been detected, the one made according to the known art and the other according to the present invention, which were submitted to extreme working conditions.
A first sample, sample A, was made according to the traditional art in one panel made of planks of red fir, the thickness of which was of about 8 mm. The second sample, sample B, was made according to the present invention, overlapping two panels P1 and P2 each one with planks, made from red fir, with a thickness of about 4 mm.
Both samples A and B, having a dimension of about 500x300 mm, were each glued on a respective maple frame with a thickness of about 34 mm, and were submitted to the same pre-conditioning step in the cell, during which they were maintained for 6 days at the conditions of 20+2° C. and 35+5 relative humidity, and to subsequent controlled heating steps in a stove at following temperatures:
- exposures for 8 hours at 30+2°C.;
- exposures for 7 hours at 35+2°C.;
- exposures for 8 hours at 40+2°C.;
- exposures for 15 hours at 45+2°C.;
- exposures for 23 hours at 50+2°C.
Between one exposure and the subsequent one, the samples A and B were left to cool.
During the exposures, at predetermined time intervals, the main parameters of each sample were measured (weight in grams, percentage change with respect to initial weight and length) and the general conditions of its appearance (presence or absence of bulges and/or cracks).
However, it was found that a first through crack, extended along the entire length of sample A, occurred at a junction between two adjacent planks after 15 hours of exposure at 45+2° C. With equal conditions, for sample B only a small bulge was observed, at a junction between one plank and
another.
Concerning the sample B of the sounding board, a first non-through crack in one of the two panels that compose it, was observed after 19 hours of exposure at 50+2°C., when the through crack of the sample A of the sounding board was now extended.
Due to extreme working conditions, as it is obvious, it is clear that in a sounding board according to the present invention the likelihood of the occurrence of cracks is dramatically reduced if not completely set to Zero....
... it was found that a sounding board according to the present invention advantageously allows the generation of a sound of a better quality with respect to that which can be obtained, at the same conditions, with a traditional sounding board.