New, Used, Sanded, and Misused
We decided to do a couple of photo experiments to demonstrate how different rosin starting techniques actually affect the rosin.
Pictured above are four cakes of rosin. They are all the same brand and model in four different conditions. All four rosins began in the same, new condition before our experiment.
This is a brand-new cake of rosin.
It has just been removed from the packaging.
The surface of the new rosin is unmarred--it is smooth and shiny.
Many people see a new rosin like this and think there is no way that a powdered form of rosin can possibly be pulled from this pristine prism of potential stickiness.
This is a rosin cake that has been scuffed and scratched with sandpaper.
As you can see, the surface of this rosin has changed quite a bit. When applied to a bow, the sound with this rosin was okay, but a little grainy.
As can easily be seen, there is rosin everywhere.
The table was sticky. Our fingers were sticky.
The camera was, unfortunately, sticky, too.
(This was a messy process.)

This is a rosin that has been scraped rather unceremoniously with the (button) end of a bow screw.
You can see small divets and grooves in the surface of the rosin.
You can also see small flakes and chunks of rosin just sitting on top of the surface.
We tried to avoid getting to the edges of the rosin cake while trying this technique so that larger bits of rosin didn't fracture and break off of the top or edges.
It wasn't easy to avoid and, as a result, pieces of rosin came off the edge.
When applied to a bow, this rosin sounded grittier and less focused than the other rosins we included in our experiment. Additionally, the bow hair caught on the broken edge and did not glide easily along the surface of this rosin.
This technique is also rather sticky with the added bonus of small hunks of rosin flying all over the table and floor during the "prep" process.

This rosin has been used normally on a violin bow. Nothing was done to this rosin
before it was used on the bow. We did ten passes of the rosin (up and down) along the hair of the bow.
The surface of the rosin is scuffed from the normal application alone and the bow was ready for playing. The hair was uniformly covered with now-powdered rosin.
This rosin produced a warm, but clear sound and was easily controlled on the strings.
With a few more regular applications on a bow, the surface of the rosin will be
leveled and the remaining shiny spots will be scuffed, too.
Additionally, there was no cloud or cascade of falling/flying rosin with this technique.
The hair moved smoothly across the rosin because there were no grooves,
cracks, or fractures created by sandpaper or a bow screw.