Conservation work on the Peter H. Raven Library’s copy of The Orchidaceae of Mexico and Guatemala has begun! Follow along as we document this painstaking restoration of one of the largest and grandest volumes in the Garden’s rare book collection.

What Is Alum?

Alum is an aluminum salt that has been used for hundreds of years to process skins, dye cloth and make crisp pickles.

Alum also has a long history of use in European paper production. During the period of hand paper-making (Middle Ages to about 1800), small amounts of alum were added to gelatin size to make the size stiffer and more water resistant. The alum in use during this pre-industrial age was mined out of the ground or extracted from shale and slate through a process of purification that involved roasting, soaking, mixing with potassium hydroxide and evaporating. The aluminum salt of this era took the form or either aluminum potassium sulfate or aluminum ammonium sulfate.

Aluminum Sulfate

In the 19^th century, all of this changed. A new process for manufacturing alum was invented. It involved treating bauxite with sulfuric acid and produced a salt that took the form of aluminum sulfate, which cannot be purified in water because it is too water soluble. Unpurified aluminum sulfate often contained excess acid and iron.

This change for the worse in alum production coincided (unfortunately) with the increasing use of alum in paper production, primarily as a component in alum-rosin size.

As we discussed in the previous blog, hand-made paper sheets could be dipped in warm liquid gelatin size, but paper made in long, wide ribbons on machines needed a different sizing method. In 1807, a German named Moritz Illig proposed treating wet paper fibers with alum and then adding rosin soap. The alum would act as a mordant, causing the rosin to stick to the paper fibers throughout the paper-making process, until a final heat treatment melted the rosin into the paper. Alum-rosin size became standard in machine-made paper in the 19^th century. All alum is acidic, but in the form of aluminum sulfate and in larger quantities, it is especially destructive.

Is Bateman’s Paper Alum-Rosin Sized?

We have previously described the poor condition of the paper in our copy of The Orchidaceae of Mexico and Guatemala by John Bateman. We have speculated that the cause of this poor condition is likely due to the paper having been machine made and alum-rosin sized.

Bateman’s paper looks machine made, and it failed a gelatin test. These factors are all pointing to our being correct in our speculations. Now we will test for the presence of alum.

Even hand-made gelatin-sized papers often contain alum as a size hardener, but we are looking for alum in large quantity. As a point of comparison, we tested a Whatman filter paper (pure cellulose) and some hand-made paper along with small pieces from each of the 3 kinds of paper in Bateman—thin cover, medium text weight, and heavy lithograph paper.

We used a testing method described by Cathleen Baker in her book, From the Hand to the Machine. The ammonium salt of aurintricarboxylic acid is dissolved in water and applied to the papers. The color results are interpreted as follows:

Pale pink—negative

Raspberry—aluminum

Purple—iron

The color differences are subtle, but the change from light orange/pink to dark raspberry took place in about 5 minutes on the small piece of cover paper. The color change was clear. The 2 controls did not change color and remained a light orange/pink.

Again, in about 5 minutes the color changed to dark raspberry on the text paper, but the color change on the lithograph paper was even more dramatic. It turned so dark that we may want to consider the presence of iron as well as alum.

Conclusions

As with the previous gelatin test, we must bear in mind that this test is limited and conducted on only 3 small pieces of paper. It would be ideal if we could obtain confirmation of these results via more sophisticated, non-destructive means, but as it is, all observations, chemical and visual, are consistent with these papers being machine-made and alum-rosin sized. We can assume for now that our speculations in this regard are likely correct.

What do these results mean for our copy of The Orchidaceae of Mexico and Guatemala?

Alum-rosin sized papers can be stabilized by washing and deacidifying, but we have decided not to use these treatments on our Bateman due to the possible harm they could cause to the books’s color lithographs (see part 2 of the Bateman blog).

Instead, we will have to rely on proper handling and environmental conditions for the long-term preservation of this book. These are important lines of defense for all books, but are especially important when other, more intrusive means of chemical stabilization are not used.

Learn more about the restoration of the Bateman book:
Part 1 | Part 2 | Part 3 |Part 4 |Part 5 | Part 6 | Part 7