It’s Book Soup!  “Sou-sou” is a toning medium made by boiling down paper degradation products into a thick sludge, and then letting the sludge harden into little cakes that can be used like watercolors to tint tissue repairs with an “antique” hue.  Theoretically, the evaporation process concentrates naturally occurring carbonates in the water, producing a final material that is pH neutral (we take issue with this claim later), and is also completely reversible.  We’re not sure where the term “sou-sou” comes from (if you know, please leave us a comment).  A description of this toning medium was written up by Piers Townsend in his article “Toning with ‘paper extract'” in The Paper Conservator, Vol. 26 (2002) 21-26.

Like many methodologies in book & paper conservation, this process is more akin to craft than science.  There are no hard and fast rules about what quantity of old, acidic woodpulp paper should be used with respect to how much water; in Townsend’s article and in conversations with other conservators, crafters of sou-sou seem to go by their instincts rather than set measurements.  The following description of our process at ISU is therefore intended as a practical but general guideline, rather than a specific recipe.

Eight acidic volumes dating from the 1880s through the 1950s were selected from the Library’s stash of discards.  Several of these textblocks were already in pieces.  We removed the textblocks from their cases, trimmed spines on the guillotine, and separated the pages.  Our bonus ingredient was several highly acidic record sleeves from the 1960s.  The acidic raw materials were divided into three batches.  A large (30″ x 25″) washing tray was filled with about a half-inch of deionized water, and one-third of the raw materials were submerged in the bath.  The materials were left to soak approximately 1 to 2 hours, while the bath was agitated periodically.  The soaked materials were then removed, and another third of the raw materials was added to the same bath water.  The process was then repeated a third time.  We ended up with over 5 L of “soup” the color of apple cider, which we filtered into clean containers.

Townsend specifies that the wash water may have a low pH, but that naturally occurring carbonates will become concentrated during evaporation, rendering the final product pH neutral.  He also claims that the bath water should not contain alkalizing agents, as these can cause the brown coloring to precipitate out of solution.  The interns and I were uncomfortable with these assertions.  None of us could understand how the simple act of boiling could neutralize an acidic starting pH, and we also saw no problem with the brown coloring precipitating, since all the water would eventually be boiled off, anyway.  Therefore, we decided to separate our soup into two batches, one of which we would make according to the article’s directions, and the second of which we would alkalize before boiling.  Test Batch 1 was used straight, while Test Batch 2 was brought to pH 7.0 by adding approximately 12 g CaCO3 to 2.5 L of soup.  (I would have preferred Ca(OH)2, but the calcium carbonate is what we had on hand.)  We let the two batches sit overnight in the refrigerator, during which time a great deal of the CaCO3 fell out of solution.  We decanted and used the saturated solution, discarding the precipitate.  A test with pH strips before we began the boiling process indicated that Test Batch 1 had a pH of approximately 4.0, while Test Batch 2 had a pH of approximately 7.0.

We boiled the soup in two batches over high heat, until it had reduced to a gravy-like consistency.

The gravy was then simmered over slightly lower heat, with occasional stirring, until it had reduced to a caramel-like sludge.

We reduced the heat to low and continued to cook the caramel, stirring constantly and watching it carefully to avoid burning.

When the sou-sou had reached the consistency of thick pudding, we poured it out into porcelain watercolor trays.

The trays were then set back on the heat unit on the lowest possible setting and left to dry out until the cakes had hardened completely.  When the sou-sou was ready to use, we performed two basic pH tests.  First, we used a sable brush to apply recalcified water directly from our water system tap onto a pH strip.  Then, we used the recalcified water to reactivate Test Batches 1 and 2, and the colorants were also applied to pH strips.

Even acknowledging the slight color distortion caused by applying colorants to the pH strips, the difference in pH between Test Batch 1 and Test Batch 2 were undeniable.  Test Batch 1 is clearly still acidic.  We then tinted a strip of white, Western, machine-made paper with each batch of sou-sou, let the samples dry, and tested them with a pH pen.

Again, the results were undeniable: Test Batch 1 showed a yellow line, indicating an acidic pH, while Test Batch 2 showed a purple line, indicating a neutral or alkaline pH.  We were pleased with the hue of our final product, and look forward to using it to tone solvent set tissue and other repairs.  However, in future we will always include the step of neutralizing the soup before we begin the boiling process, in order to end up with a truly non-acidic product.