About how many pounds will glass add?

[TroyH]

I'm building 2 simple red oak board bows for me and my son.  Will kind of be matching.  Anyway, I am probably going to back them with fiberglass cloth, but I'm wondering about how many pounds that will add.
I don't want to make him a 30# bow and then add glass and have it turn into a 50# bow.

[Pat B]

Why add glass at all. If you are worried about grain violations add a cloth(linen, silk) backing or a simple brown paper bag will work and none of these will ad draw weight. If you want to add draw weight put a hickory or maple backing. Glass can overpower the belly wood.     Pat