Spider Silk

Spider Silk (SS) is the protein fiber created by spiders for the purposes of crafting webs, immobilizing prey and protecting egg sacs. One of the strongest naturally occurring materials known to man, this product is used frequently as a reagent in Britannian and  Serpent Islander magic.

Britannia
Spider silk lends properties of binding to spells which incorporate it, although in order for it to act as a magical component, a full ounce of the material is necessary. Originally this reagent was harvested only from the albino ghoul spider, although later years it appears that other species were employed, and caches of webbing could often be found in the nests of common giant spiders.

In addition to serving as a magical catalyst, the webbing of giant spiders may also be employed as thread or fish line, and communities living in areas where other fibers are hard to access, such as Sir Cabirus's Abyssal Colony, have often used spider silk to such ends.

Serpent Isle
Erstam reports that some mages, seeking to overcome the difficulty of finding sufficient naturally occurring quantities of the reagent for magical practice, had taken to raising giant spiders domestically on farms, that their silk might be harvests. No evidence of such operations, however, were found during the Avatar's journey to the Serpent Isle, and similar endeavors have failed on Earth, given the propensity captive spiders have for cannibalism. 

Trivia

 * According to the group responsible for producing the world's only known extant textile made of spider silk, it takes over fourteen thousand spiders to produce a single ounce of silk, making the presence of giant spiders in Ultima's setting rather serendipitous. The real world tapestry, woven from the silk of over a million  golden orb spiders, weighed over 2.6 pounds.