Ballast is a standard piece of equipment pilots have in their model box for those windy, turbulent days. By increasing the mass of the glider, ballast helps the glider fly at a faster optimal speed thus increasing range, as well as calming down the glider to not be so influenced by the turbulent air it flies through.
Common materials used for ballast includes:
Of the three, lead is most common as it is cheap, easy to find, and comparatively heavy. We do not recommend using lead due to health issues related to touching lead. Brass is a nice material to use, and is also quite economical to use, and we recommend using it for weights up to around 40 grams. Much more above that, the ballast stick becomes too long, which spreads the mass too far from the center of gravity, increasing the glider's moment inertia (no good!).
Tungsten is the premium material to use, due to its extremely high density. Due to its higher price-tag, we recommend using tungsten for your heavier ballast sets.
Below is a simple way of building your own ballast sets, using 1.5 mm music wire, a good set of pliers, a wire cutter, and your choice of weights.
Make a 90 degree bend in the end of the music wire to retain the weights, and slide the weights onto the wire.
Jog the wire directly in front of the weights to hold the weights in place.
This is a very important step. Remember this is just one way of doing it, and the background is that I prefer flying with a slightly-moved forward CG when I insert ballast. Take your stick of weights and place the center of the weights at the CG location. Mark the wire where the retaining hole will be located, and bend the wire 90 degrees 5 mm behind that (towards the weights). Then mark the depth of the retaining hole to the canopy to the wire, and carefully make a smooth 180 degree turn.
Cut the wire to a suitable length, so that it will not pop out unexpectedly when the canopy is in place.
And you're done! Make several different weights, and fly them in as many different conditions as you can to gain a peek into the black magic of ballast.