Weight drop on bows due to moisture?

[Aksel]

I got quite a chock today. Last night I put a layer of home made pine pitch lacquer on a bow I havenīt used since summer. Itīs a yew bow which I then weighted at #48@27.
I put a string on it and it seemed surprisingly light when I pulled it so I got my bow scales out. It measured #35!!

I live in the Netherlands where winters are humid so I store my bows on the top (2nd) floor of my house where I assume it is drier then below. Only thing is my washing mashine and tumble drier are close by and I donīt ventilate as often due to insane gas prices here.... A bow dropping #12 in a 3 months seems alot but I assume it must be the damp winter combined with keeping them next to the tumble drier that caused it.   

I obviously have to move the bows somewhere. I wonder if I should take the lacquer off or just move them and check if they return to weight before I remove any finish. Unfortunate I just put pine pitch lacquer on all my bows! I donīt dare checking the other bows and I wonder what the draw weight of the #48 recurve I just finished will be in summer  I also assume they are not safe to shoot since moisture must have gone up a few %... 

Puh!!

At least I learned a couple of things. And a few layers of tung oil doesnīt stop bows from picking up moisture!

How do I find out which room is driest and most suitable to store my bows?

[Pat B]

Do you have central heat in your house? If so place the bow(s) near one of the vents where the heat comes out. If the bows took on moisture with the finish on them they should also lose moisture with the finish on them.
 You can build a simple hot box to keep your bows from taking on moisture with foam board or even cardboard and incandescent light bulbs.

[bradsmith2010]

yes the moisture from the drier,,,

[Aksel]

Pat: Yes I do have central heating in the house but way too many bows to store near one went so thatīs why I wondered how to think in general about how to find the driest room. I live in an old house and because insane gas prices heating is off this winter in all rooms which weīre not using. Hot box could be an idea but not for long time storing of 30 bows plus. I suppose I can dry a few bows out at the time at a spot with warm air flow.

My issue with the finish is that the bows soaked up all this moisture treated with tung oil only, and yesterday I put all this lacquer on in one swoop on most of my bows without realising about the moisture problem, so in worst case I sealed all this moisture in.

I put one bow on some digital scales and will check it daily to see if itīs dropping physical weight.

bradsmith2010: yes most likely 

[Zugul]

from my very little understanding it's pretty much impossible to block completly the moisture exchange of a bow, no matter what kind of seal you apply. In worse case scenario it will take a long time to lower its moisture content, but if you put it in a dry enough place over time the moisture will drop.

[darinputman]

If you can find a finish that will do anything besides slow the taking on of moisture let me know as I would like to use it. Best I can do is slow it down. Wouldn't worry at all about having applied another finish, just do as some of these good folks have suggested and should be fine. Good luck and enjoy.   
     

[Aksel]

 I start to suspect that the staves were well below 9% moisture content in the summer when I made the bows and now theyīve hit equilibrium at around 10% and are weak.

Example: one yew bow dropped from #48 (unknown physical weight) in summer to #35 (434grammes) in winter.
48h in hot box and itīs gone down to 430g physical weight and draw weight is up to #38.

Question:

Is there a formula for how draw weight increases as moisture content decreases?

[George Tsoukalas]

 I have never seen such a formula.

 But I have also had the problem of moisture gain causing loss of draw eight. New Hampshire has dry winters, the time when I try to make bows, and them comes very humid summers resulting  in loss of poundage. My bows for the years kept in heat during the winter and AC in the summer but it still happens because moisture is tough to control.

Such are the challenges  of wooden bowyers.

Jawge

[bradsmith2010]

osage is resistant,, your variation seems pretty extreme,, maybe the scale is part of the difference,,maybe you measured differently in the summer?

[superdav95]

Ya there isn’t any way to seal completely unfortunately.  The 2 part epoxy finish might be the best out there for slowing it down from what I hear but even then it still happens.  Even wooden bows with fg laminations still lose a bit.  12 pounds does seem quite a lot to lose though.  I would keep near a dry heat source for a bit or keep in hot box like you’ve done. 

[Aksel]

Bradsmith2010: Itīs yew. and, yes itīs supposed to be resistant to at least string follow with high water content. And it hasnīt taken any set because of me drawing it a few times. I did use the same scales to measure it and I am absolutely sure it has dropped massively in weight.

Jawge: I just found a formula by Tim Baker. "wood stiffens 5-6% for every 1% it dries.

With my basic math skills: (I think the bow was about 43# when I made it late last spring) #43 at (a hypothetical) 9% moisture.


Say it dries down to 7% in summer.  #43X1,06=45,6 x1,06 =#48,3 (which is the draw weight the bow had in the peak of summer.

Now lets assume it gains 5% moisture in winter, so it ends up 3% higher in moisture than what it originally had when I made it at #43.

With the same formula I land on #35,7 calculating backwards 3% which also seems reasonable for the atlantic climate here in the Netherlands. 80 RH now

makes sense when I imagine it this way

But does this formula make sense? Think itīs from TBB4

[George Tsoukalas]

Makes sense, Aksel. I wonder where Tim got it from. I bet he got it from his vast experience. Tim knows his bow stuff. Jawge

[bradsmith2010]

you right Jawge

[Jjpso]

You need to get a dehumidifier! That’s how I solved the problem. Placed my bows in a room that is not used and placed a dehumidifier there. I have a hygrometer and the moisture is always fine

[superdav95]

You need to get a dehumidifier! That’s how I solved the problem. Placed my bows in a room that is not used and placed a dehumidifier there. I have a hygrometer and the moisture is always fine

This makes sense actually.  Just store them in a separate room.  Good thinking.  I’m gonna try this.  When done shooting them back in the room with dehumidifier