When I was in middle school I was told to scratch my rosin. Today I see many teachers saying the same thing to their students. This needs to stop. Scratching rosin looks useful because when you scratch rosin with sand paper you see a lot more powder come off of the block. When I explain to my students why they shouldn't I relate it to a block of chalk. If you scratch chalk with sandpaper, or with a paperclip before using it more chalk with definitely come off, but none of the scratched powder will adhere to the blackboard. I decided to contact some of the major string companies to figure out what they say to do.
Andrea (One of the most highly regarded rosin companies in the world) has on their site:
It is not necessary. Just simply start rubbing at the frog of the bow with a bit more pressure and you will see the shiny surface giving up and powderising. Make sure you don't put too much rosin or it will cake up on the strings.
A customer service rep from Pirastro sent me an email stating:
You do not have to scratch our rosin before each use…and also not if the rosin is new.
A representative from the makers of Jade, and Millant rosin stated in an email:
No it's no use to scratch it before. If your rosin is not old it should work normally. But when your rosin is getting old and dry it might seem to be more difficult to get it on your bow. Don't keep rosins longer then about 2-3 years if you want to be sure.
If that is not enough, a representative from D'Addario stated:
There's no real need to scratch the surface of rosin - it will deposit itself onto the surface of the bow hair naturally.
Scratching your rosin is not hurting anyone, but it is not helping either. It is about time we stop perpetuating the belief that scratching rosin is normal, and start getting the most use of our favorite cakes of rosin.