Clear glass marbles are a great choice for filling up volume in a small fermenter or oak barrel.
Clear glass marbles, free of ink or contamination, are an ideal solution. Use just as much as you need to top up the vessel. When you transfer the beverage out, remove your marbles, wash and reuse with your next batch.
3 lbs of marbles is enough to displace approximately 1 Liter of liquid or 1/4 gallon.