Hi Joan, I have always had the same problem whether painting with (recent) glazes or ala prima. Dust and fibers! After reading your post it dawned on me what my problem may be... I store my brushes upright in a variety of cans and jars. They look like furry flower arrangements. I have tried storing them in a case, but I like to SEE all of my brushes at once. All I need do is look around at my silk plants and see the dust settled on them to realize what is happening. From now on I will give my brushes a bath before beginning a session. I will also cover them with a lightweight fabric, such as a scarf between sessions.
I too have carpet in my 'studio' It is my plan to put wood on this floor, but until then I will refrain from vaccuming while a painting is drying (bummer, I looove to vaccum
).