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Sizing Terminology and Practice

Winter 2015
Winter 2015
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Timothy Barrett is director of the University of Iowa Center for the Book and an associate professor at the University of Iowa School of Library and Information Science. He received a BA degree in art communications from Antioch College in 1973. His career includes two years at Twinrocker Handmade Paper, Inc. in Brookston, Indiana, two years under a Fulbright fellowship studying papermaking in Japan, and two years of part time study at Western Michigan University¹s Department of Paper Science and Engineering. His many years of research on early European handmade papers have been funded by the NEA, the Kress Foundation, the Institute for Museum and Library Services, and as of 2010, a MacArthur fellowship. Barrett is author of two books, one website, seven videotapes, and thirty-six articles or book chapters on the history, technique, and aesthetics of hand papermaking. His primary research interests are in early European papermaking technology, the role of gelatin in paper permanence and the aesthetics of paper. Specialized handmade papers produced by Barrett and his students are designed for use by conservators of rare books and works of art on paper, fine press printers, and book artists.  When I was taking courses part time at Western Michigan University's School of Paper Science and Engineering in the early 1980s, one of my professors, Ray Janes, gave us a succinct description of sizing that has been useful to me ever since: "Sizing is a material added to paper to change its resistance to the penetration of a liquid." That was it. The definition works for all types of paper, whether made by hand or by machine. Thus, while sizing is useful for keeping ink from bleeding and is essential in a watercolor paper, it is also crucial in making a whipping-cream carton that can keep the cream inside where it belongs.  There are two key classes of sizing—"surface" and "internal."

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Surface sizes are added to the sheet after it is formed and dried. Other terms used to refer to surface sizing include tub sizing (because the paper was dipped into a tub of warm gelatin) and external sizing (because it is sometimes applied to the exterior of the sheet with a brush or rag). Historical surface sizes include wheat or rice starch as employed by Islamicworld papermakers, or gelatin, commonly used in Europe between 1300 and 1800 and to the present day in some watercolor papers. In addition to providing liquid resistance, surface sizes also typically increase folding endurance and resistance to soiling and abrasion. Surface sizing (like increased beating time) tends to lower tear resistance because it decreases the amount of work needed to drag longer individual fibers from the main web-like structure of the sheet. The fibers tend to break off instead, with a decrease in resistance to tearing. Internal sizes are added to the pulp before the sheet is made. When the sheet dries, it is sized, that is, it is much more resistant to the penetration of liquids compared to a "waterleaf" (unsized) sheet. Other terms for internal sizing include "engine sizing" (because it was often added to the beating "engine" or Hollander beater). The first internal size, rosin–alum, came into common use in the nineteenth century. The two components of this sizing together resulted in a finished paper that was acidic and therefore more likely to degrade over time. Papers made with a finished alkaline pH tend to last much longer. Therefore modern cellulosereactive internal sizes have been developed that can be used with a calcium-carbonate alkaline reserve added to the pulp. This yields an alkaline and sized paper. These materials include both alkyl succinic anhydride (ASA) and alkyl ketene dimer (AKD) sizes. Aquapel and Hercon are examples of AKD sizes. There is another term, "vat sizing," that is sometimes used that should be addressed here. It is a confusing term because it can be used to describe the process of surface sizing paper by dipping into a vat or a tub of warm gelatin solution, or it can also refer to internally sizing paper by mixing a sizing agent with the pulp in the papermaking vat before sheet forming. The two recommended terms, surface and internal sizing, are clearer. Depending on their needs and goals, hand papermakers today have a range of materials and application methods to choose from to create a desired effect in their paper. To begin with, hand papermakers can "size" their paper, without the addition of sizing, just by increasing beating time. Macerating the fiber for longer periods of time often results in a paper that is tougher, stiffer, and more resistant to the absorption of liquids. But increased beating time also tends to produce a paper prone to higher shrinkage, which can be problematic. Sizing is a handy and flexible ingredient in the papermaker's tool kit that can increase the range of finished paper characteristics. For example if you are making a large wall piece and need the finished paperwork to dry flat, you might beat the fiber just long enough to form acceptable sheets. Later, if you want to apply watercolor or do a sumi-ink drawing on the surface, you can brush on a surface size to control absorption of the aqueous medium and strengthen the work for wall hanging. Remember that sizing does not need to be applied to the entire area of the sheet. True, by definition, internal sizing is normally applied to the wet pulp so that the entire finished sheet is sized, but what if you made a waterleaf sheet and then sprayed or brushed internal or surface sizing onto the sheet only in particular areas related to the planned image? As an example, an artist may surface size a collage element, adhere it to a base sheet, then brush the piece with a black-walnut-dye solution.The dye would bead up on the surface-sized collage area, but bleed extensively into the unsized areas of the work. Consider using internal sizing in the sheet followed by surface sizing. For a papermaker fabricating watercolor papers, the internal sizing might keep the surface sizing from soaking in, thereby causing it to remain more on the surface of the sheet, possibly giving enhanced performance with watercolor or other media. To select the right sizing agent for a project, here are a few of my recommendations. If you want an entire batch of paper to be more resistant to liquids, using a commercially available cellulose-reactive sizing is probably the easiest and best way to go. Sizing sold by Carriage House and Twinrocker will fill the bill. Following the Carriage House application procedure, add 30 milliliters of sizing per 450 grams of dry fiber. Note that it may take several days for the sizing to fully activate. You can also use a moderately hot iron for a minute or two on a surrogate sheet to see whether your application rate is giving you the liquid-resistant properties you are after. If you want to strengthen a paper or make it more resistant to soiling and abrasion (in addition to increasing the resistance to liquids), then I would recommend surface sizing with gelatin. There are many grades of commercial gelatins that are available: acid and alkali extracted, photo grade, pharmaceutical grade, and food grade. Cow (bovine) bone and pigskin are common raw materials for modern gelatin manufacture. Historically, parchment clippings, rabbit skin, other bits and pieces of four-footed animals, and fish bladders have all been used to make glue or sizing for papermaking. I know of no research that has definitively shown a particular type or brand of gelatin to be superior for conservation or hand papermaking. At the University of Iowa Center for the Book (UICB), we use a photography-grade, bovine-bone gelatin produced by Gelita AG ( Carriage House offers a similar or identical gelatin for papermaking. Or one could begin experimenting with Knox unflavored gelatin purchased in your local supermarket in the baking-supplies section. When we at the UICB want to make a paper tougher and stronger, we typically prepare a 3% gelatin solution (30 grams in one liter of water). We add the dry gelatin to the water overnight, then heat the solution up in a double boiler. This can be done without a double boiler but it requires more attention to keep the solution from burning at the bottom of the pan. We heat the solution until the gelatin dissolves but not much more. The solution should be warm or very warm to the touch, but not hot—about 50 degrees Celsius (or 122 degrees Fahrenheit). We add to the gelatin solution: 1% potassium aluminum sulfate based on the weight of the dry gelatin by first dissolving the alum in 100 milliliters of water. The alum "hardens" the gelatin, which is said to increase resistance to liquid penetration and may leave the paper more resistant to future microbial attack.2 Research by Irene Brückle shows that alum applied at a 1% rate does not harm gelatin-sized papers subjected to accelerated aging.3 To apply the gelatin sizing, dip your paper into the warm solution, then stack the sized sheets on a board covered with a felt designated for sizing, add a felt on top, and press lightly. If the sheets are small, you can stand on the top pressboard for 1 to 2 minutes. Thereafter, separate the sheets promptly, to prevent sticking, and hang them to dry. The entire sizing operation should take no longer than 5 to 10 minutes. For papermakers averse to working with animal products, alternative external sizes include wheat or rice starch, or egg whites. The Northeast Document Conservation Center offers a good overview of starch or methyl cellulose paste preparation methods.4 To use these preparations for paper sizing, experiment with diluting paste consistency with water. Place handmade paper on a sheet of blotter and apply sizing with a wide brush such as a 4-inch paintbrush. Move the sheet aside until the sizing has lost its stickiness, then flip the sheet over and apply the sizing to the opposite side. The same method can be used with gelatin sizing as well. In addition to starch sizes, egg whites are sometimes used in Islamic-world papermaking. Traditional recipes call for alum addition, however, an alternative non-acidic egg-white size can be prepared by employing a technique used by Center for the Book lettering artist Cheryl Jacobsen when she needs a binder to prevent gouache from offsetting in a codex. First, separate the yolks from the whites. Two extra-large eggs give about 75 milliliters of whites, so plan accordingly depending on your needs. Whip the whites with a whisk or eggbeater for 3 to 5 minutes until a fine foam is worked up, then pour everything into a glass. Cover with clear plastic wrap, and place in the refrigerator to stand overnight. In the morning the foam will sit on top and a yellowish clear liquid will remain at the bottom. Remove most of the remaining foam with a spoon and pour off the clear solution to use as sizing. As of this writing, we do not know if sizing made without alum in this way is more or less resistant to inks or possible future microbial attack. Regardless of sizing type, after the sheet dries it is likely to be wavy or curled. To flatten, spritz the sheet with a spray mister, just enough to slightly moisten. Move the sheet to a piece of interfacing material (Pellon or blotter) and continue to build a stack of moistened sheets, and end with a sheet of interfacing on top of the pile. Wrap the stack in 2-mil or similar sheet plastic and let stand 1 to 3 hours until the sheets have relaxed, then load them into a stack dryer. Take care to moisten the paper only enough to make them relax, otherwise the sizing may become tacky and stick to the blotters in the drying system. If this becomes a problem—rather than misting the sized paper—it may prove helpful to moisten a set of Parted damp sheets are set out on mesh racks to dry. If the sheets are especially tacky or wet with size they should be turned over repeatedly until the size cools and loses its stickiness. Photo: Carolina Larrea, 2005. blotters instead and then place the sized sheets between them before wrapping the stack in plastic to humidify. Sizing, like many traditional papermaking techniques, holds great potential for contemporary papermakers and paper artists. The best approach is to investigate and try out the traditional methods first. Once they are learned, they become a solid reference point from which exciting and promising innovations can grow. ___________ notes 1. William J. Barrow Research Laboratory, Permanence/Durability of the Book— VII: Physical and Chemical Properties of Book Papers, 1507–1949 (Richmond: W.J. Barrow Research Laboratory, 1974). For more, see http://paper.lib.uiowa .edu/chron.php#gelatin (accessed July 28, 2015). 2. W. J. Barrow Research Laboratory, Physical and Chemical Properties of Book Papers, 1507–1949. Permanence/Durability of the Book 7 (Richmond, Virginia: W. J. Barrow Research Laboratory, 1974). 3. Irene Bruckle, "The Role of Alum in Historical Papermaking," The Abbey Newsletter vol. 17, no. 4 (September 1993), byorg/abbey/an/an17/an17-4/an17-407.html (accessed July 28, 2015). 4. NEDCC offers the leaflet online at preservation-leaflets/7.-conservation-procedures/7.3-repairing-paper-artifacts (accessed July 28, 2015).