The Studio blog
Yes, it can!
I was getting my bow re-haired at Kenmore Violins recently and was able to ask the shop owner that very question. He told me that rosin does indeed become less effective if it dries out. How long a block of rosin remains useful has to do with the quality of the rosin and the climate you are in. High quality rosin is pure. Rosin is the solid form of resin acquired from conifers, and cheaper brands (that often come with rental instruments) can contain less than 50% pure rosin in them.
So here is what I learned:
If your rosin is leaving a thick, white layer of dust on your instrument the rosin is not pure.
You should never have to scratch up the surface of your rosin in order to apply it.
You wouldn’t need to use a lot of high quality rosin, a little goes a long way.
Be mindful of where you buy your rosin from. Large retail music stores likely purchase rosin from suppliers in bulk, and the one you are buying could very well be 10 years old by the time it gets to you. Your local violin shop/luthier will probably have rosin to sell that they order in smaller quantities so what you buy is newer.
You get what you pay for, so the more you are willing to spend on a block of rosin the purer it is likely to be.
(D’Addario produces quality rosins that are not very expensive and a good option for beginning students).
If you leave your rosin in a hot, dry climate it will become dry and brittle, so keep it with your instrument. You are keeping that in a stable climate, yes?
And finally, how do you know if your rosin has gone bad? The rosin should be soft enough that when you draw your bow hair across it, a dusty path will be left on the block, (as shown in the image). If you do that and the block remains shiny or there is not any dust left in the bow’s path, then your rosin has hardened and is no longer of good service to you.