Here is Nick Pearson's recipe for making watercolor paper: INGREDIENTS Cotton linters (American "Buckeye" is his favorite) Rag (He uses dry lap half stuff, imported from India by Khadi Papers; In the US, Cheney half stuff is a good equivalent) Alkyl ketene dimmer (AKD) for internal sizing (He uses Aquapel) Titanium dioxide (TiO2), also known as Titania, used to increase opacity Wet strength resin (WSR) for increasing the paper's surface strength (He uses Kymene that he obtains from his local papermill) Food-grade gelatin for surface sizing (He uses Knox) FIBER PREPARATION Pearson finds that watercolor paper made solely from linter pulp feels too weak and limp, and that watercolor paper made solely from beaten rag is too wild and lumpy. Mixing the two fibers—50 percent rag and 50 percent linters—makes for a good compromise. The goal is to obtain a moderately well hydrated, fairly short pulp. Beat the fibers separately to control the fiber length and hydration. Beat the rag fibers until they are around 1.3 millimeters in length with a drainage measurement of 80 SR; and beat the linter fibers until they are around 0.9 millimeters and 60 SR.1 ADDITIVES Pearson uses additives to achieve an opaque, "hard-sized" watercolor paper on which watercolor paints will ball up on the surface rather than soak in. AKD is an internal sizing agent. Rag seems to need more sizing than linters. Be aware that unless the paper is dried using heat, AKD takes about two weeks to cure. For rag add a 6-percent solution AKD (6 milliliters AKD to 100 grams dry-weight pulp) and for linters add a 2-percent solution AKD (2 milliliters AKD per 100 grams dry-weight pulp). Pearson's technique for adding the AKD and most other additives is to take a liter of pulp from the beater at the end of the beating cycle, mix the AKD into that, blender it up, put it back into the beater, then run the beater for a few more minutes. WSR (wet strength resin) is an epichlorohydrin. It forms bonds between itself and the fibers to act like a glue when the paper is re-wetted. It also prevents the fibers from becoming detached and balling up under the paintbrush. It is an irritant and your skin can become sensitized to it after repeated contact so use rubber gloves (even though concentrations in the vat are low enough not to be a worry.) Add 2-percent WSR to both rag and linter pulps.2 Titanium dioxide (TiO2) is a filler. It adds opacity to the finished sheet. Add 1-percent TiO2 (1 gram per 100 grams dry-weight fiber). Pearson adds this to his stock chest rather than into the beater. Retention aid. Add appropriate amounts of retention aid if the AKD and WSR require one. SHEET FORMATION Pearson's goal is to produce paper that weighs between 250 gsm (grams per square meter) and 300 gsm.3 While forming sheets Western style, maintain a vat consistency of at least 1.4 percent (1.4 grams of dry-weight fiber in 100 milliliters water). To do this, re-charge the vat with the equivalent quantity of pulp after pulling each sheet. The sheets are thick, so to speed the draining Pearson places the mould on a vacuum table and sucks out excess water before removing the deckle. He couches onto rough woolen felts to give the paper a nice texture. The paper is wet pressed and then dried under tension in a restraint drying system. GELATIN SIZING The AKD creates a water-repellent surface, but the paper is still porous and needs to be surface sized to assure good holdout of the pigment or dye.4 Gelatin sizing fills the pores in the paper and needs to be done before the AKD cures. Mix a 5-percent solution of food-grade gelatin in hot water (5 grams gelatin in 100 milliliters water). It requires a lot of stirring! Pour this into a tray that sits inside a second, larger tray, filled with boiling water and keep the gelatin solution above 50 degrees Celsius. If the gelatin drops below 50, it will set and will not size properly. Dip a dry sheet of watercolor paper into the gelatin tray and leave it there for a minute or so. Carefully lift it out and, using clothespins, hang it on a line to drip dry. Pearson turns the sheet several times and uses a little sponge brush to gently remove any streaks or drips. DRYING & PRESSING When the paper is still damp but the wet sheen has gone, Pearson runs the sheets through a photographer's drum dryer or cylinder dryer, which dries the paper under heat and tension. The paper is inevitably a little warped coming off the drum dryer, so he dry presses it by sandwiching the sheets between felts and leaving them in a standing press for a few days. If you do not have a drum dryer you can let the sheets air dry. They will cockle, but you can re-dampen them and re-dry them, either between felts or in the restraint dryer. The result is a beautiful, cream-colored, hard sized sheet of watercolor paper with impressive holdout. The crisp rattle of the sheet and the deckle edges set it a cut above the standard watercolor papers. Pearson reiterated that this recipe is by no means the only way to make watercolor paper. Like all good recipes, it can handle a bit of experimentation and variation. In fact, Pearson recently wrote me that if he could find a good source of linen rags he would like to try adding them into the fiber furnish, as he thinks this would further decrease the porosity and improve paint holdout. For his customers, and those of us learning from his experiments, this will be something to look forward to. 1. In Pearson's opinion, anyone who is using a beater and not measuring fiber length and SR is effectively flying blind. Fiber length is often measured by pouring a standard amount of pulp through a mesh screen then weighing the fiber. Pearson uses seven plastic knives, spaced apart and taped together, to gather the wet fibers from the beater for weighing. Fiber length is usually a relative measurement rather than microscopic analysis. SR is a measure of wetness made using a Schopper Riegler drainage-time testing device. The SR measurement for unbeaten linter is around 15 SR, unbeaten half stuff is around 35 SR, rag pulp for currency paper measures around 40 to 50 SR, and "overbeaten" pulp measures around 90 SR. Pearson runs his beater at 1.7-percent consistency, at 10 degrees Celsius, with 7.2 kilograms on the beater arm. Under those conditions, Pearson provided these average SR values for running his beater using rag half stuff: 0.0hr 30 SR, 0.25hr 45 SR, 0.5hr 55 SR, .75hr 60 SR, 1hr 65 SR, 1.25hr 72 SR, 1.5 hr 75 SR, 2hr 80 SR 2.5hrs 85 SR. Thus his recommendation for rag pulp at 80 SR represents a long beating time. For reference, a 1.5-pound (680-gram) dry-weight fiber load in 12 gallons (45.4 liters) of water will beat at 1.5-percent consistency. 2. Pearson uses the wet strength resin to give the finished sheet more surface strength so that fibers do not "ball up" when being worked with a wet brush; it is not an essential ingredient. 3. For reference, an A2 sheet weighs 62 grams at 250 gsm and 75 grams at 300 gsm. 4. The term "holdout" refers to color retention at the surface rather than penetration into the sheet, resulting in a washed-out appearance.
