After weaving, some rugs are sheared little, and left with a thick full pile. Others are sheared very thin with a low profile pile. However, it's almost inevitable after many years of use, some rugs will begin to show "knot heads."
Below: A hand knotted, piled rug, is woven with individual knots: Flat on the reverse side, with loose fibers on the face.
Below: Through heavy use or sometimes even types of dyes (depending on the rug), knot heads may begin to show. Areas which previously had a higher profile may wear away, slowly exposing the sometimes shiny "head" of a knot on the face of a rug. Notice below how the outlined "Pile" section has a higher profile, hiding the base of the knot. The area outlined with "Knot Heads" demonstrates a lower pile height, thereby exposing the top of the knot which is referred to as the Knot Head.
Below: So long as wear is very evenly distributed throughout a rug with no foundation showing, knot heads may or may not adversely affect value providing all else equal. Some purists and collectors may find rugs showing no use more desirable than others.
While older rugs in immaculate condition may fetch higher premiums, it is generally accepted there are individuals who place high
decorative value on carpets which exhibit knot heads throughout.
Often, the appearance of knot heads across the face of a rug may
enhance the overall luster of a piece, perhaps even adding to the
"authenticity" of an seemingly older example.
As demand for rugs with an "older feel" has increased, there have been several processes weavers and importers have subjected rugs to: Some of which include Golden, Tea, and Herbal washes. While these rugs may augment the overall patina and aesthetics, they do not address the pile height. Within the last 15 years or so, there has been a very large increase of newly imported goods from weaving countries which have been subjected to age-progression. It is not entirely uncommon for newer rugs to be sheared very thin to mimic the feel of older rugs. In extreme instances and to add to the "authentic" look of an older rug, it's not unheard of that a newly woven carpet may be subjected to heavy foot traffic pre-exportation, sometimes beaten, or even left in streets to be driven upon by cars and trucks. It's not unheard of for such rugs to be intentionally torn in select areas to then be repaired soon after. There are several other closely guarded secrets on how new rugs may be subjected to age-progression to further replicate older rugs which may involve chemicals, abrassive brushing and other.