A starting runecaster may choose to master runes in exchange for initial equipment. The Runecasting supplement is a little bit vague about how runecasters can master additional runes, however, and says nothing about restrictions on the number of runes known.
Here is the actual text:
A starting runecaster may choose to master runes in exchange for initial equipment. After that, runes must be gained through adventuring. Although runes are a written type of magic, a character cannot learn how to use a rune just by studying one. There is much more to mastering a rune than simply writing it. The runecaster must have a living example of the rune – whether from an instructor, a power rune, or divine intervention. Learning runes from an instructor requires a teacher of higher level than the character and typically involve a high cost and/or some form of service (a “special quest”).
So as per the rules, you can’t study a ‘normal’ rune to master it. A runecaster can learn from a power runes, however (like a magic-user learning a spell from a scroll in AD&D). A character must study a power rune for at least a full day to master it and the power rune is destroyed when used in this manner.
The Runecasting supplement also mentions that an instructor can teach a student new runes. Without constraints, this would mean that every apprentice runecaster would simply teach his fellow students the secrets of the runes he has mastered until every rune was known by all. However, teaching somebody the secrets of rune magic has its costs. Like creating a power rune, it requires one day, 100 gp, and 50 XP from the instructor.
In addition, a runecaster is limited to a number of known runes equal his level +3 (generally equal to his Lore modifier). No take-backs, either – once you master a rune, you can’t forget it. Learning a rune is kind of like getting a tattoo; it’s permanent, so choose wisely
These rules prevents rampant copying of runes by apprentices and acquisition of an excessive number of runes at low levels, while allowing runecasters to gain runes more easily by adventuring (via power runes).
Dwarven Glory 