Someone said something about 'muslin, not cotton'. Muslin usually IS cotton. SOME artificial fibres do odd things in boiling water, but other that that, you can use anything thin enough. The 'tea ball' idea (if we mean the same thing) is probably a good one too. The ones I have used are 2 halves, wire-mesh, hinged together, when closed about the size of a table-tennis ball, and hangs by a short chain. I gather that there are other shapes.....