1) If it is a tufted rug – it may the the glue that is used on the backing of the rug. Once the glue dries out, it starts to break down and leaves a large volume of fine dust. The proper way to correct this is to use a latex glue on the back of the rug and then cover the latex with a cheese cloth or felt.

2) Fine oriental rugs with a tight knot count can hold dust deep down in the rug between the warp and weft of the rug and the base of the face fibers.

The dust looks like black sand – it is really dirt that as you walk on the rug breaks down and gets trapped deep in the rug. Most good rug cleaning companies vibrate the rug (face down) to remove the fine dirt before it gets cleaned. You can get much of the dirt out by vaccuming the rug upside down – lower the beater bar to the lowest level. This may take several vaccumings.

Thanks!