Sizing

Part of Paper Making

Applying size to paper to control ink absorption — why unsized paper is unusable for writing and how to make effective sizes from available materials.

Why This Matters

Unsized paper acts like a sponge. Drop a single drop of water or ink onto it, and it spreads instantly in all directions, following fiber channels deep into the sheet. Writing on unsized paper with any liquid ink produces illegible, feathered results — the letters bleed together into fuzzy blobs, fine details disappear, and ink may bleed through to the reverse side.

Sizing fills the spaces between fibers with a coating that controls absorption. Good sizing allows ink to sit on the paper surface long enough to dry in place, producing clean, sharp lines. Different sizing materials and concentrations produce different effects — from lightly sized paper that accepts ink slowly but still shows some feathering, to heavily sized paper where ink sits almost entirely on the surface and dries sharp and crisp.

For a rebuilding community, sizing is what makes paper useful as a writing surface rather than just an absorbent material. Every sheet destined for permanent records, maps, books, or official documents should be sized. The additional processing time and materials are a small investment compared to the alternative of illegible records or wasted ink.

The Science of Sizing

Unsized paper has a porous microstructure. The fiber mat contains millions of tiny channels and spaces that draw liquid in by capillary action — the same mechanism that draws water up a paper towel. The driving force is surface tension between water and the cellulose fiber surface.

Sizing works by:

1. Filling pores: The size material physically fills small pores, reducing the volume of space available for liquid to flow into
2. Changing surface chemistry: Many sizing materials change the surface from hydrophilic (water-attracting) to slightly hydrophobic (water-repelling), reducing capillary force
3. Creating a surface film: A thin continuous layer on the paper surface slows ink penetration by requiring it to break through the film before entering the fiber structure

Different sizes achieve these effects in different ways and degrees. The goal is not to make the paper completely water-repellent (which would make it impossible to write on at all) but to slow absorption enough that ink dries before it can spread significantly.

Animal Glue Size

Animal glue — made from collagen extracted from animal hide, hooves, and connective tissue — is the most effective traditional sizing and was used for European writing paper from the 14th century onward.

Making Animal Glue Stock

Raw materials: Cattle, sheep, or pig hide scraps; hooves; ear cartilage; knuckle joints; sinew. The more connective tissue, the better — lean meat contributes little collagen.

Process:

1. Cut raw material into small pieces. Remove any fat or flesh (fat inhibits gelatin formation).
2. Cover with cold water and soak overnight. This draws out blood and loosens surface material.
3. Drain, replace with fresh water, and heat slowly to 60 to 70°C. Do not boil — boiling degrades collagen and produces weaker gelatin with an unpleasant smell.
4. Hold at 60 to 70°C for 2 to 4 hours, stirring occasionally. The liquid will become pale amber and increasingly viscous.
5. Strain through fine cloth to remove solids.
6. Cool the strained liquid — if it sets to a firm gel at room temperature, the collagen concentration is adequate. If it remains liquid, continue concentration by gentle simmering.
7. Dried and stored gelatin: pour the gel into shallow dishes, allow to set and partially dry (a few days of air drying in a warm space), then break into pieces and store dry. Dried animal glue stores indefinitely and dissolves in hot water for future use.

Target concentration for sizing: The stock gel should be approximately 5 to 8 percent protein. Test by making a small amount into a solution and dipping your finger — when pulled away, a thin, barely visible film should form. Too concentrated and the sized paper will be stiff and shiny; too dilute and it will not size adequately.

Applying Animal Glue Size

Working temperature: Keep size solution at 40 to 55°C during application. Below 35°C, gelatin begins to set and cannot penetrate paper evenly. Above 60°C, gelatin degrades.

Dipping method (best for even coverage):

1. Heat size to 45 to 50°C in a dipping vessel slightly larger than your sheet size.
2. Hold each sheet by one corner and lower it smoothly into the size, ensuring it is fully submerged.
3. Hold for 5 to 10 seconds while moving slightly to ensure complete penetration.
4. Lift and allow excess to drip back into the vessel.
5. Hang on rope or lay on drying rack. Do not overlap — sheets will stick together while drying.

Brush method (when dipping vessel is unavailable):

1. Lay sheet flat on a clean surface.
2. Apply warm size with a broad, flat brush. Work quickly — gelatin sets fast on cold paper.
3. Apply strokes in one direction, then the perpendicular direction.
4. Flip sheet and repeat on the reverse.
5. Hang to dry immediately.

Drying: Sized sheets are fragile until fully dry — handle by corners only. In warm weather with good air circulation, drying takes 1 to 2 hours. In cool, humid conditions, 4 to 8 hours. Do not rush drying with excessive heat — too-rapid drying causes warping and can crack the size layer.

Starch Size

Starch from wheat, rice, corn, or potato is an effective sizing material, especially for lighter writing papers and for calligraphy with brush inks.

Making Starch Paste

1. Mix starch powder with cold water in a 1:10 ratio (1 part starch, 10 parts water).
2. Heat while stirring constantly. At approximately 60 to 70°C, the starch will suddenly thicken — this is gelatinization.
3. Continue stirring and heating for 5 minutes after gelatinization.
4. Remove from heat. The paste should be the consistency of thin soup when hot — it will thicken somewhat as it cools.
5. Strain through fine cloth to remove lumps.

For sizing use, dilute further: the final sizing solution should be slightly thinner than the freshly cooked paste. Too thick a starch size produces a surface that is somewhat sticky when damp and may cause sheets to stick together in humid weather.

Application: Starch size is applied by brush rather than dipping, as it does not penetrate as freely as warm gelatin. Brush onto both surfaces. Allow to dry completely. Light burnishing after drying improves surface smoothness.

Performance comparison: Starch size provides somewhat less water resistance than animal glue but is adequate for most writing purposes. It is preferred for brush calligraphy because it creates a surface that holds the ink just below the surface rather than completely on top — a quality prized in Chinese and Japanese writing.

Alum and its Role

Alum (potassium aluminum sulfate, or similar aluminum salts) is frequently used in combination with other sizing materials to improve performance. Alum can be found naturally as a mineral (alunite) or produced as a byproduct of certain dyeing processes.

With animal glue: Adding a small amount of alum to animal glue size (approximately 1 part alum to 4 parts glue by weight) improves the water resistance of the sizing and makes the surface slightly more resistant to mold growth. The alum partially crosslinks the gelatin, creating a tougher surface film.

Caution: Alum is acidic. Paper sized with alum-containing formulations will gradually acidify over time, causing yellowing and embrittlement. For archival documents intended to last centuries, prefer pure gelatin size without alum, or add a small amount of chalk (calcium carbonate) to the pulp to buffer any acidity.

Vat Sizing vs. Surface Sizing

All methods described above are surface sizing — applied after the paper is formed and dried. This gives the most control over sizing level and allows different batches to receive different treatments.

Vat sizing adds sizing material directly to the pulp in the vat before sheet formation. The sizing is deposited throughout the sheet as it forms. Starch is the most common vat size; some rosin-based sizes also work in the vat.

Vat sizing is faster (no separate sizing step) but harder to control — the sizing distributes unevenly if pulp is not well-stirred, and the concentration in the vat changes as sheets are pulled, requiring constant adjustment.

For a small community mill, surface sizing offers better control and more predictable results. Reserve vat sizing for bulk production of paper where precise sizing level is less critical (wrapping paper, packaging).

Testing Sizing Effectiveness

After sizing and drying, test before committing a full batch to writing use:

Ink test: Write a line of text with your primary ink. A well-sized sheet shows:

• Clean, sharp edges on each letter stroke
• No feathering or spreading
• No bleed-through to the reverse side
• Ink dries within 15 to 30 seconds

Water drop test: Place a single drop of water on the sheet surface. A well-sized sheet holds the drop as a bead or slowly absorbs it (30+ seconds). An under-sized sheet absorbs the drop immediately. An over-sized sheet repels the drop so strongly that writing is difficult.

Folding test: Fold a small strip of sized paper and press firmly. If the size layer cracks along the fold line (visible as a white crack in the surface), the size is too heavy or was applied over-concentrated. Reduce concentration in the next batch.

Size Failure and Remediation

Sheet remains absorbent after sizing: Size was too dilute, or sheets were not fully submerged during dipping. Resize with a slightly more concentrated solution.

Sheets stick together after sizing: Too much size applied, or sheets were stacked before fully dry. Dry completely before stacking. Reduce size concentration.

Surface has a shiny, almost lacquer-like appearance: Size concentration far too high. This surface is too impermeable — writing inks may bead and not adhere. Wipe with a barely damp cloth and re-test.

Mold growth on sized paper in storage: Size provides a food source for mold in humid conditions. Dry storage is the primary prevention. Adding a small amount of alum to the size reduces mold susceptibility. Copper-containing additives (copper sulfate, available where copper is worked) have strong antimold properties — add a trace to sizing solution for paper stored in humid climates.

Sizing transforms paper from an absorbent material into a writing surface. It is not optional for any paper intended for permanent written records — it is the final step that determines whether your community’s knowledge infrastructure will actually work when it is needed most.