WHICH ROSIN TO CHOOSE
Rosin is made from sap - also called resin - collected from pine trees - much in the same way as maple syrup from maple trees. Sometimes sap from other trees, such as larch, spruce, or fir, are mixed in to create a specialized formula. The sap is purified by straining and heating until the resins are completely melted. When it is cooked, the concoction is poured into molds and after it sets for about 30 minutes, the rosin is smoothed down and polished. Finally, the Rosin is packed into a cloth or fitted into a tight-sealing container.
The color and consistency of rosin is determined by the time of the year it is harvested.
.....Gold / Amber: harvested late winter or early spring. This lighter rosin has a harder consistency and is good in summer but also all year round. Violin and viola use both llight and dark rosins.
.....Dark brown: harvested summer or fall
This rosin has a softer consistency and is usually best used in the winter and in cool, dry climates. It is mostly too sticky for hot and humid weather. Cello and bass use darker rosins.
.....Student-grade rosin: is cheaper. It often has a grittier sound and produces more powder than the professional grades. This could be an advantage for some playing styles, f.ex. fiddling.
.....Professional-grade rosin: is more expensive. It is made from a purer resin and generally produces a smoother, more controlled tone.
.....Powdered rosin: is sometimes used by the string shop to get the bow hair started after it has been rehaired.
.....Special formulas: Some companies also add precious metals to their recipes. These materials purportedly increase the rosin's static friction, creating different sound qualities. Metals used include: gold, silver, copper, and more.
Under the microscope: To make a string vibrate so it can produce a sound, friction is needed. When a string is plucked, the finger provides this friction - one pluck at a time in slow motion. The horse hair has by nature a rugged surface but it's really the stickyness of the rosin that creates most of the friction.
Other uses of rosin: Rosin is also used in ballet as anti-slip on shoes, and in sports, f.ex. to create more secure grips.
(Based on articles in Strings magazine, 2004 and Wikipedia)