In the mid-eighth century, the Arabs learned techniques of paper manufacture from the Chinese. The oldest Greek paper manuscripts were produced during the ninth century. Paper (carta or charter) was made in Muslim Spain beginning in the late eleventh century. During the twelfth and thirteenth centuries, it was used in Italy and the Mediterranean for merchants' notes and by notaries for registers; from the thirteenth century on, paper was actually manufactured in Italy. During the fourteenth and fifteenth centuries, production spread to Switzerland, the Rhineland, and France. In England there was limited production in the fifteenth century; only in the mid-sixteenth century was the paper making industry permanently established. (In the late fifteenth century, the famous publisher William Caxton and his colleagues were still largely importing supplies from Italy and France.) Correspondence was often written on paper beginning in the fourteenth century, and paper was commonly used in low-grade books from c. 1400 and in legal documents from the sixteenth century (although parchment also continued to be used). Ruling on paper generally consists of frame ruling only. The humanists (see Humanistic) revived hard point ruling for a time, which worked well for parchment but damaged paper. In general, ink or lead point was used for ruling paper codices. In early paper books, quires are often protected by parchment outer sheets or guards. Paper was traditionally made from cotton or linen rags, although more exotic substances such as silk were often employed in the Orient. The rags were soaked and pulverized until reduced to a pulp and were then placed in a vat with a solution of water and size. A wooden frame strung with wires (producing horizontal laid lines and vertical chain lines) was dipped into the mixture and agitated until the fibres fused to form a sheet of paper. This was then placed between sheets of blotting paper and pressed. The paper produced was then either trimmed or left with its rough (deckle) edge. Paper frames often incorporated wire devices (in the form of designs or monograms), which leave an image in the paper known as a watermark. There exist reference volumes containing reproductions of watermarks from broadly datable or localizable contexts, and it is frequently possible to identify watermarks by matching them against such reproductions. Early paper is generally quite resilient, but beginning in the mid-nineteenth century, when book production increased dramatically, wood and other organic pulps were used (either completely or as additives). These substances introduce a level of acidity into the paper which causes it to turn brown and eventually to crumble away, presenting great difficulties in preservation. Modern acid-free papers are now available.