A C19th discussion of the Persian lustres in the Godman collection (now in the British Museum) illustrated a lustre sherd that had been found at Rayy, just south of modern Tehran, and one of the largest cities in Persia, situated on the crossing points of major North-South and East-West trade routes. This sherd was called a "waster" and thus the attribution of Rayy as the producing centre for lustre became widely accepted. Watson, however, disputed the identification of this sherd as a waster, saying that the term has been much abused as an indicator of local production, and that spoiling during firing is quite common. Only the presence of a very large number of spoilt pieces at a site can imply local manufacture: finding one spoilt piece is much less significant. Furthermore, a vessel is not necessarily unsaleable if it has flaws such as distortions of walls, splashes of coloured glazes from other vessels, or jagged pieces from adjoining vessels that have fused into the glaze during firing, as in the case of this published lustre sherd. The majority of these flaws occur during the first firing, yet this lustre sherd was still considered valuable enough to decorate with lustre and refire.

The evidence for Rayy as the producing centre is, without this "waster", very slim, as is the evidence for the other postulated centres of production of Persian lustre a Sava (just south-west of Rayy) and Jurjan (north-east of Rayy, on the shores of the Caspian). A large number of pristine lustre vessels was recovered from Jurjan in the mid-1940s, found packed in large jars and buried. It seems, however, that instead of being local products these were part of the stock of a merchant, which was horded and buried at the time of the Mongol invasion in the 1220s. One of these vessels even bears an inscription reading "The work of Muhammad ibn Muhammad al-Nishapuri al-muqim bi-Kashan", ie. living in Kashan. No direct evidence of local production (wasters, kilns etc) has been found at any of these suggested production centres, nor is there any literary evidence or nisbas mentioning Rayy or Jurjan.

In contrast, there is a wealth of evidence from a variety of sources pointing to Kashan as the place of production. The geographer al-Muqaddasi, writing c.985, mentions even then that Kashan was reputed for its ceramics, and Yaqut (c.1220) praises Kashan’s ceramics and says they were exported far and wide. Even the Persian word for tile derives from the city’s name: "kashi" or "kashani". The remains of kilns and wasters have been found here, though the majority of evidence derives from the pots themselves which have an abundance of dated and signed inscriptions on them, and giving the nisba (ie. the home-town of the potter) "al-Kashani". It is significant that the potter mentioned above, whose nisba is Nishapur, takes time to stress the fact that he is working in Kashan. This must have given him some credibility in the potting community!

The first person to identify Kashan as the dominant production centre in the region was Ettinghausen in his 1936 article in Ars Islamica. In this article, he isolated a series of diagnostic motifs, all secondary elements of the design, from pieces which were attributed to Kashan through inscriptions; he postulated that these unimportant features, drawn without thought, were unlikely to vary or be imitated and thus, like signatures, were a clear way of identifying a hand or workshop. [To view these diagnostic features, go to: Common Motifs in Kashan Pottery]. From this beginning, he identified a number of previously unassigned pieces to Kashan workshops.