Point 1: foundation
the foundation of a carpet would refer to the knot count. most oriental carpets’ knots are counted based upon a knot per square inch count. one would simply count the number of knots from an inch measured from the width, multiplied to the knot count measured one inch along the length. a per square inch count would result.
one can also examine the various knot tying techniques, ranging from the persiansennehknots, to the turkishghiordes knots, to the double tibetan knots. there is also another category of carpets that are flat, with no pile. these are kilims (a generic term meaning flat woven rug), soumak weaves, and needlepoint’s and aubusson weaves are similar.
a high knot count does not mean the carpet is the best. one must consider the other following points.
Point 2: material content
an oriental carpet is made of three parts, the warp, weft, and pile, unless it is a flat woven rug. in a woolen carpet, the warp, or main structure of the rug is usually made of cotton, silk, and or wool. the first two mentioned are preferred, since the material does not expand or contract due to environmental influences. the pile of the carpet would be made of wool, silk or a mixture. then there is the weft line, which is the same as the warp. after every row of knots woven, a weft line is drawn in between the warp threads, so that each row of knots is secure.
flat weaves, (also know as weft faced rugs), aubusson weaves, and soumak weaves would only have the warp and the weft lines. soumak weaves have an added dimension.
one can go into detail of the material origins. wool from different parts of the world makes a difference in the price point, not to mention silk. there is an issue of dead wool (from deceased sheep) verses live wool, which would affect the amount of natural oils in the wool. these elements would affect the wear of the carpet. if the material used in a carpet is inferior, even with a fine foundation, the carpet life expectancy is not too long.
for most people let your hands do the looking.
feel the carpet during inspection. this is a very tactile product. some even would try to walk barefoot.
Point 3: design
most oriental designs are remakes of remakes of the first design that appears in a carpet. but, overall, one must examine the design execution. the following questions must be asked: how well was the pattern drawn in the carpet? how well was the nagsheh (blueprint of the design) was drawn for the weavers to follow in producing an oriental carpet? is the design symmetrical or asymmetrical?
some older persian designs were not exactly made perfectly, especially in the rustic tribal designs. the errors one may find in these designs are called “the persian flaw.” in the weaver’s eyes, only god was perfect.
Point 4: colors
the choice of colors within a design is also important. it is not uncommon to find one design to have two to four versions, with different color combinations. other criteria to examine, is to see how well the translation of the colors are done in the new production from the original. one could have the most detailed design but one bright color accent seems to ruin the entire feel of the carpet. then the question of the type of dyes used in the carpet must be considered as well. most oriental rugs made after world war ii will have some form of a chemical based dye, in order to achieve a certain color. aniline-based dyes were used over natural vegetable dyes. however, early aniline-based dyes were a bit unstable until other chemical bases were used, such as chrome-based dyes. but, unlike vegetable dyes, the colors would not soften over time, which is the general preferred effect. vegetable dyes cost more to be used in rug production today. many new rugs are now going back to traditional practices of making an oriental carpet with natural vegetable dyes.