Bending Ivory:
As to the ancient methods for softening, forming, and re-hardening elephant ivory, most of the older books read almost identically to this typical entry from "The Scientific American Cyclopedia of Receipts, Notes, and Queries", edited by Albert A. Hopkins, published by Munn & Co., 1892, page 290:
Ivory, to soften:
1.) In 3 oz. of spirits of nitre, and 15 of water, mixed, put the ivory and leave for three or four days.
2.) To make Ivory Soft and Flexible. -- Take a solution of phosphoric acid of 1.130 sp. gr. Put the ivory in this solution, and let it remain until it has a transparent appearance. Take out, wash carefully, dry between soft linen. The ivory will be soft as thick leather. It will become hard if it is exposed to the air, but become soft again if placed in warm water.
Softening and Hardening Ivory
Each piece of ivory has its own reactive potential and therefore the following information is for informational purposes only and is offered without warranties or guarantees of any kind. The information presented here is based solely on Mr. Stevens own testing and experience.
SAFETY NOTE: This procedure utilizes phosphoric acid. Always wear eye protection and plastic or latex gloves when handling acids. Also, always pour acid into water and never water into acid. Pouring water into acid can cause a boiling and splashing reaction.
Softening Ivory
Make a solution of water and phosphoric acid with a combined specific gravity of 1.130. You will need a hydrometer and a hydrometer test jar (tube) for this procedure.
Let the ivory soak in this solution until it loses its opaqueness and takes on a semi-transparent or translucent appearance.
If soaking a thin panel, the ivory may want to curl. Prevent this by placing a weight, like a heavy clear drinking glass, on the ivory while it soaks.
Remove from the solution and wash thoroughly in clean cold water to stop the acid reaction and then dry it off.
Ivory will be flexible and possess the approximate hardness of thick leather.
The ivory will gradually harden with exposure to the air, but immersion in simmering hot water will restore its softness and flexibility as long as the ivory has not been properly dried yet in decrepitated salt.
Actual effects of this procedure:
The following information is based on a 1/16th inch thick panel of ivory that is 1 ½” wide and 2 ½” long. Variations will exist for ivory pieces at different thicknesses and sizes, though the variations will be minor unless the ivory is very thick.
A 1/16th inch thick panel of ivory soaked for 5 hours becomes flexible enough to wrap around a 1/16th inch diameter rod. Soaked for 7 hours, the ivory is so flexible, it can actually be folded back onto itself without splitting.
Ivory soaked for 5 hours reacts to carving tools like thick soggy leather and spreads badly as the carving tools are pressed into or through it. For carving purposes, soaking for only 2-to-3 hours softens the ivory but leaves enough strength in the ivory structure for more accurate carving with less “spreading” of the ivory as the tool pushes into or through the material.
Any serious softening will cause the ivory to spread at least a little under the pressure of a carving tool, so care does need to be taken when carving softened ivory. Softening an ivory surface slightly by using a little water is more often a method used for carving as it does not cause the ivory to spread under tools and if dried right away, will usually cause no permanent damage to the internal structure of the ivory.
After soaking, the ivory retains its softness and flexibility for approximately 24 hours before it starts to stiffen. At the end of that 24-hour period, the ivory will still not be hard and remains flexible, just not quite as flexible as it was right after soaking. It remains flexible and simply does not regain its original hardness and structural integrity until after it is dried in decrepitated salt for at least 24 hours. It can take up to 48 hours to harden completely if the ivory was soaked for a more extended period of time.
Drying Ivory After Softening
To restore ivory to its original color and hardness after softening, wrap the ivory in a sheet of white paper and cover it with dry, decrepitated salt and let it set covered for 24-to-48 hours, depending on how long the ivory was soaked.
Clamp thin panels of ivory wrapped in paper between flat pieces of wood or other stiff material while drying to keep the ivory from re-hardening in wavy curves.
Making decrepitated salt:
Method #1:
Spread common table salt out on a baking pan and dry it at very low heat in the kitchen oven until it loses its crystal appearance and takes on a dense opaque white look. Since this method takes a while, leave the oven door open to prevent the oven from getting too hot inside and burning the salt.
Method #2:
Spread a 1/2" deep layer of Epsom Salt out on a baking pan. Bake in the kitchen oven at 400 degrees Fahrenheit for an hour and it will lose half its water content and be as good as dried table salt as a desiccant.
At 480 F for two hours the Epsom Salt will lose virtually all its water weight and becomes one of the best desiccants available.
Put into fine-weave cloth bags, or sealed containers. Remove as much air as possible from the containers when sealing or closing. Weigh your full containers. When the containers gain 25-to- 50% more than their original weight, the salt has rehydrated and it is time to recharge the Epsom Salt’s desiccating ability by baking it again.
SAFETY NOTE: All Epsom Salt you buy is fully hydrated. The reason is because if you add water to dried Epsom Salt, the salt will get VERY HOT. So be careful to not spill water on it or place it in direct contact with wet ivory. Its is important to dry your ivory very well before wrapping it in white paper and covering it with the dried Epsom Salt.
Of interest :
"Ivory and the Elephant: In Art, In Archeology and In Science", George Frederick Kunz, 1916, p.243:
Very thin, pliant veneers have been cut into spirals out of a solid block of ivory by means of a feather-edged veneer saw, and some years ago a Monsieur Page patented a process for such work. He produced pieces 17 by 38 in., and asserted that he could make much larger pieces, up to 30 by 50 in. The excessive thinness -- 1/50 in. (note: = .020") -- and the transparency of this veneer renders it not very well adapted for application to wood, etc., as the material beneath would show through to a certain extent. As material for the painter's art, however, it might answer better.
Charles Holtzapffel, in "Turning & Mechanical Manipulation", 1846 (published 10 years before Kunz was born), Vol. I "Materials", p.154 footnote, while also mentioning this same method of producing thin sheets indicates that they had been somewhat thicker than that referred to by Kunz (Kunz' M. "Page" and Holtzapffel's M. "Pape" are obviously one and the same person, with the latter spelling probably being correct since it appears twice and predates the Kunz reference):
Monsieur H. Pape, of Paris, piano-forte manufacturer, has taken out patents for this method of cutting ivory spirally into sheets. A specimen, 17 inches by 38 inches, and about one thirtieth of an inch thick (note: = .033", slightly over 1/32"), glued upon a board, may be seen at the Polytechnic Exhibition in Regent-street, and M. Pape advertises to supply sheets as large as 30 by 150 inches (note: this must be a misprint, as 150" would exceed the workable length of even the largest tusks; probably Kunz' 50 inches is the correct number). He has veneered a piano-forte entirely with ivory. Similar veneers of ivory are now also cut in England.
I would suggest that much thicker sheets could be produced if the tusk section (or at least the material on its outer surface) was first softened before being cut. As support for this idea, Kunz also mentions the impossibly large pieces found in some very ancient statuary (pp.22-23):
The very large size of the pieces of ivory which must have been required by the Greeks in the production of their colossal gold and ivory statues, some of which were forty feet or more in height, the face, hands, and feet being of ivory, and even the large size of some of the consular and other diptychs that have come down to us, have raised the question, how did the ancients secure pieces of ivory of sufficient size? In our day, with the processes now in use, this would not be possible. Hence it has been conjectured that they possessed some lost art for welding together separate pieces of ivory. In the late Latin treatise on the arts of the Romans, belonging to the tenth century, and which passes under the name of Eraclius, the following directions are given: "Take sulphate of potash, fossil salt, and vitriol; these are ground up with very sharp vinegar in a brass mortar. Into the mixture the ivory is placed for three days and nights. This being done, you will hollow into a piece of wood as you please. The ivory being thus placed in the hollow, you direct it and will bend it to your will." But this recipe as well as others given by various ancient writers do not give satisfactory experimental results.
Although in some instances tusks of quite exceptional size have furnished very large flat pieces of ivory, it is regarded as possible, if not probable, that the ancients cut large cylindrical pieces from the median part of the tusk, split these cylinders at a given point, softened them by some process, and then flattened them out, thus securing a piece as broad as the circumference of the tusk. This theory was advanced by Mons. Quatremere de Quincy. Steeping in vinegar and almond oil does really render ivory ductile to a certain extent. While it can be decomposed by caustic alkalis, it cannot be recomposed.
