Often times "fines" in a sample contain a higher concentrations of precious metals than the overall sample. Therefore, if you loose some of the fines, you will lower the overall assay of the precious metal in questions. This can be especially true with samples which contain a large percentage of moisture, where the liquid that drains off can contain fine particles.
For this reason, it is important to use the appropriate sample bag.
Often times, the type of seam can have as much, or more, impact as the "weave" of the bag. A folded seam will provide a much tighter seam than a "turned" seam.
You can review various fabrics and types of sample bags here:
This is a type of bag that will allow water to drain, but have a very fine pore size so less fine material is lost. It has been suggested that fine grained gold can be lost when water drains from other types of porous bags. The "micro-por" or Micro-Shield type bag is a latex coated woven polyester bag. It is also referred to as a "Bone Velour" bag. These bags drain moisture very slowly. Look for the bags marked as "Micro Shield" on this page:
I didn't know that about the gold loss from some types of bags - that's interesting. I'll pass that on to a couple of my buddies and see if they know anything about this. Thanks for the link to the micro-por type, no doubt they will want to check it out.