No scientific records here, only a subjective - albeit very strong - impression.
I have in my home “workshop” two harpsichords in construction:
- A Zuckermann/TPW French double kit. 2x8’, 1x4’. Two keyboards. 63 notes from F to g’‘’.
- A made from scratch big flemish my design derived from Ruckers but of course with modifications and strenghtenings to withstand 5 octaves. 2x8’. One keyboard. 61 notes from F to f’‘’.
They are both in more or less the same state: case done, still no bottom, the two soundboard made from the same wood (abete della Val di Fiemme, I think it’s named spruce in English) from the same planks. Both have their soundboard still not glued into the case.
However, n. 2 has the soundboard furniture glued (bridge, ribs, boudin), while the n. 1 is thinned but without ribs and bridges and boudin/4’ hitchpin rail).
The last difference is n. 1 is still bare wood, while n. 2 has been entirely sized up and down sides with very diluted hide glue.
When you finger-tap, or when you just caress the two soundboards (put inside the case but not glued into it yet) with your fingertips, n. 2 responds with a lively sound, very loud, deep tone, different at the right of the bridge than at the left, at the tail than just behind the gap.
N. 1, on the other hand, responds with a thinner sound, more acute, much less loud, shorter (this I am not entirely sure but seems shorter. Impossible to say when just tapping or caressing).
I haven’t measured the effect, I’d not even know how to, but it’a strong. I mean, it’s not a vague impression: the two sounds are very different. I had already registered this effect previously, but now I can compare having at the same moment two harpsichords at my disposal. I believe this effect is given by the hide glue sizing. I’ll tell you if the n. 1 will change its sound characteristics after I’ll size it with hide glue as well.
Lastly, this has been discussed by Grant O’Brien, I think in his Ruckers boom but who knows where. It could be Martin Skowroneck in his “Cembalobau”, sorry I am not precise but I can search if needed as I have both books at home.
Now, O’Brien - or Skowroneck - says that Ruckers probably didn’t size their soundboards with hide glue, because he (O’Brien) put his wet finger on the soundboard and didn’t feel it sticky as expected.
Well, I am sure I have sized my soundboard with hide glue, but be assured my wet finger doesn’t feel it sticky, neither.
Other famous candidates for historic sizing are: gumarabic, shellac, egg’s white. I have used shellac before, and it does keep the soundboard clean and does provide a good support for decoration, but hasn’t any consequence on sound, like hide glue has (a very good one).