Sizing, Grain and the Paste Pot

A watercolour sheet is not a neutral carrier. It is sized, it has a grain, and it remembers water. When you paste one into a book, the sheet asks the press, the grain and the paste pot three separate questions, and the cockling you see at the fore edge is the answer to all three at once. A 300 gsm sheet cockles in a case because the paste adds water to one face only, the grain direction fights the spine, and whatever sizing the mill put in the sheet decides how fast that water moves. This matters because the grain of a watercolour sheet has to agree with the spine of the book.

Which way does the grain run in a watercolour block?

Grain runs parallel to the direction the fibres settled on the wire of the paper machine. In a block, the sheets are stacked and glued on all four edges, and the grain of every sheet in the block runs the same way. You can find it without tearing anything. Bend the sheet gently in both directions: it flexes easily along the grain and resists across it. Tear a corner: a tear along the grain runs straight and clean, a tear across it wanders. Wet a small offcut and watch it: the sheet swells more across the grain than along it, so the curl tells you the direction.

Grain parallel to the spine lets the leaf turn and lie down. Grain across the spine makes a stiff, springy leaf that fights every opening and pulls at the pastedown. In a block sold for painting, the grain is usually the long dimension of the block, but you should confirm it on an offcut rather than trust the label.

Internal sizing and surface sizing are two different jobs

Internal sizing is added to the pulp before the sheet forms, usually a neutral or alkaline size such as alkyl ketene dimer or alkenyl succinic anhydride. It stops the water in a wash from wicking straight through the thickness of the sheet. Surface sizing is applied afterwards, at the size press, and it seals the top of the sheet: it holds pigment on the surface, controls how a wash sits, and decides whether a hard edge stays hard.

A heavily surface sized sheet takes a wash slowly and holds a crisp edge. A lightly sized sheet drinks fast, blooms, and lifts easily when you scrub. For book work, the surface size is the one that matters at the paste pot, because paste sits on the surface before it moves into the sheet. A sheet with a strong surface size resists the paste water for a while, then takes it unevenly. A sheet with a weak surface size takes it at once, all over, and swells fast.

How is a sheet pressed flat before it is pasted?

Pressing is the step that removes the memory of the block and the memory of the wash. The sequence we use is: damp the sheet evenly on the back with a sponge and blotter, stack it between dry blotters and a sheet of Melinex, and press it under weight overnight. A nipping press is ideal, but a board and a stack of books will do if the weight is even. Twenty-four hours is the usual figure for a 300 gsm sheet; thinner sheets need less, heavier sheets need more.

Two rules keep the press from doing harm. First, damp the back only, never the painted face, or you will move the pigment. Second, change the blotters after the first hour, then again after four, so the sheet is not sitting in its own moisture. A sheet that comes out of the press flat and slightly cool is ready. A sheet that still curls at the corners goes back in with fresh blotters.

Why does a 300 gsm sheet cockle in a case?

Cockling is differential swelling. Paste is roughly half water, and it goes on one face of the sheet. That face swells; the other face does not. The sheet cannot swell in plane because it is restrained at the edges by the pastedown and the joint, so it buckles out of plane instead. The result is a wave along the fore edge, sometimes a row of small blisters where the paste was uneven.

Weight does not prevent this. A 300 gsm sheet is heavy enough to feel substantial in the hand, but its thickness is not what resists cockling; its restraint is. Three factors decide how bad it gets. The grain direction decides which way the wave runs. The sizing decides how fast the water enters. The paste decides how much water arrives and how long it stays. A stiff wheat starch paste with a high solids content brings less free water than a thin, wet paste, and it gives the sheet time to relax.

The paste pot sets the terms

We cook wheat starch paste to a heavy cream and let it cool under water. For a watercolour plate we use it thin enough to brush but not watery, and we paste the back of the sheet, not the page. The sheet goes down onto a lightly pasted area, is rubbed through a clean sheet of paper, and is left under weight for an hour before the book is closed. If the plate is large, we paste it in strips and let each strip settle before the next.

A common mistake is to paste the whole back at once and press immediately. The sheet takes up water across its whole area in one go, and the cockling is set before the paste has a chance to dry. Working in strips, or pasting the page rather than the plate, gives the same adhesion with less water in the sheet at any moment.

Pressing, drying and the days after

A pasted plate keeps moving for two or three days. We leave the book open at the plate, under a light weight, in still air, and we do not close it until the leaf feels dry and cool to the back of the hand. Closing a book on a damp plate traps the moisture against the pastedown and prints a ghost of the wave into the facing page.

If the plate has cockled anyway, do not iron it. Re-damp the back lightly, press it again between dry blotters, and let it dry slowly. Ironing drives the sizing into the sheet and leaves a shine that will not take a wash if you ever want to work on it again. Slow drying is the only reliable correction.

What the sheet is asking for

Read the sheet before you paste it. Find the grain, test the surface size with a wet finger on an offcut, and press the plate flat before the paste pot is opened. Match the grain to the spine, keep the paste stiff, work in strips, and give the leaf three days to settle. A 300 gsm sheet will still move a little, because paper always does, but it will move evenly, and an even wave is a leaf that lies down.

Sources

• Paper Conservation Catalog, Book and Paper Group, American Institute for Conservation: https://www.conservation-wiki.com/wiki/Book_and_Paper_Group
• Hand Papermaking, sizing and grain direction: https://www.handpapermaking.org/
• British Library, care and handling of paper: https://www.bl.uk/