Cellulose Grades and Raw Materials in Fabriano Papermaking

What cellulose grade means in practice

Papermaking is fibre science as much as craft. The cellulose fibres drawn from different raw materials vary in length, purity, flexibility, and surface chemistry — all of which influence the final sheet. In the Fabriano tradition, raw material selection was central to the reputation of each mill's output, and the sorting of incoming rags was a skilled occupation distinct from the vat work itself.

The term "grade" in this context refers to the combination of fibre source, preparation method, and the resulting sheet properties: basis weight, opacity, surface smoothness, permanence against light and humidity, and compatibility with different ink types.

Cotton and linen rag

For most of the pre-industrial period, Fabriano's mills worked primarily with cotton and linen rags collected from worn textiles. These sources provided cellulose fibres already refined by the spinning and weaving process — long, clean, and largely free of lignin, the compound that causes paper to yellow and embrittle over time.

Rag sorters graded incoming material by textile type, colour, and degree of wear. White cotton rags produced the lightest, clearest sheets. Heavier linen produced denser sheets with a stiffer hand. Mixed grades resulted in intermediate products, often used for less exacting applications like wrapping and accounting paper.

Properties of common rag fibres

Cotton rag — Short to medium staple. High alpha-cellulose content. Produces soft, absorbent sheets with good permanence. Used in fine writing and drawing papers.

Linen rag — Longer fibre than cotton. High tensile strength. Produces denser, crisper sheets. Preferred for documents requiring durability over several centuries.

Hemp rag — Very long fibre. High burst strength. Common in map papers and documents subject to repeated folding.

The maceration stage

Before beating, sorted rags were wetted and allowed to rot partially in stacked piles — a process called maceration or retting. This weakened the inter-fibre bonds in the textile without degrading the cellulose itself, making subsequent mechanical separation more effective. The duration of maceration affected fibre length and therefore sheet properties: shorter maceration preserved longer fibres, while extended maceration produced more uniform but shorter stock.

Fabriano mills controlled maceration through covered pits positioned in naturally cool areas of the mill building. The temperature of the water supply from local streams influenced the rate, and experienced papermakers adjusted timing to account for seasonal variation.

Diagram showing three principal innovations of Fabriano papermakers — Schematic representation of three innovations associated with Fabriano mills: the wire mould, animal-glue sizing, and watermark wire forms. Source: Wikimedia Commons / Museo della Carta e della Filigrana.

Non-rag sources

As rag supply became constrained relative to demand in the eighteenth and early nineteenth centuries, papermakers across Europe investigated alternative fibre sources. Several plants were tested for their cellulose content and processability, including straw, nettles, and various grasses.

Alfa grass (esparto)

Alfa grass, harvested from North African and Iberian semi-arid zones, became commercially significant in European papermaking during the mid-nineteenth century. Its relatively short fibres produced sheets with a smooth, uniform formation well-suited to printing. Cartiere Miliani incorporated esparto into some grades, particularly for book papers where surface regularity mattered more than archival permanence.

Wood pulp

Mechanical wood pulp, introduced commercially in the second half of the nineteenth century, changed the economics of paper production substantially. Its high lignin content made it unsuitable for permanent documents — lignin oxidises over time, causing the familiar yellowing of newsprint — but it reduced raw material costs significantly for grades where longevity was not a requirement.

Chemical pulping processes, particularly the kraft (sulfate) process, removed most of the lignin from wood fibres and produced a stronger, more permanent sheet. Kraft pulp became the dominant raw material for most paper grades by the early twentieth century. At Fabriano, premium grades continued to use cotton fibre, while standard commercial grades incorporated kraft pulp.

Basis weight and its implications

Basis weight — the mass per unit area of a sheet — is determined partly by the fibre furnish and partly by the formation process. In the Fabriano tradition, different weights were associated with specific end uses: lightweight sheets for correspondence, medium weights for manuscript and ledger paper, heavier sheets for drawing and printing.

The relationship between fibre type and achievable weight range is not straightforward. Cotton fibre, which produces relatively bulky sheets, can reach low basis weights while maintaining adequate opacity, because the fibre mat scatters light effectively. Wood-pulp sheets at the same weight are typically less opaque because the fibres are shorter and pack more densely.

Permanence and the acid-free standard

Archival permanence became a formalised concern in the twentieth century, as conservators documented the deterioration of machine-made papers produced since roughly 1850. The standard most widely referenced today is ISO 9706, which defines requirements for permanent paper including alkaline pH, alkaline reserve, and maximum lignin content.

Cotton-based papers from the Fabriano tradition, because they lack lignin and were traditionally sized with alkaline calcium carbonate, meet permanent paper standards without additional treatment. This property is one reason Fabriano-type papers remain in use for archival and conservation purposes, including the repair of historical documents. For more on conservation standards, the Library of Congress Preservation Research program publishes accessible summaries of permanence testing methods.