Blue Amberols: A Myth that will not die

Blue Amberol cylinders manufactured between 1912 and 1929 by the National Phonograph Company (Edison) consist of a thin, blue-tinted celluloid sleeve fitted tightly around a dimensionally stable core of plaster of Paris (Wikipedia, “Blue Amberol Records”). The celluloid was intended to overcome the fragility of the older wax cylinders. An aging problem, however, has been explained by the collector community with a stubborn model that still circulates in books, articles, and forums.

The prevailing myth runs as follows: over the decades the inner plaster core absorbs moisture, swells, and thereby tightens the inner form. The cylinder then no longer fits the phonograph’s tapered mandrel. The remedy is said to be a reamer, a conical tool faced with sandpaper, used to bring the interior back to size. In severe cases the swollen plaster can even burst the celluloid sleeve. That very account appears in Wikipedia (the article “Blue Amberol Records”), and I, too, fell for this myth for a long time.

An effect can indeed be observed; there is no doubt about that. The cause, however, is a different one.

What Celluloid actually does

Celluloid is cellulose nitrate to which camphor was added from the outset as a plasticizer, typically on the order of about 20 to 33 percent by weight (Reilly 1991; Selwitz 1988). The camphor molecules are not chemically bound to the polymer (Reilly 1991; King, Grau-Bové, and Curran 2020). They can therefore migrate through the matrix and, at room temperature, can pass directly from the solid into the gaseous state (Reilly 1991).

Conservation-science studies of historic and artificially aged celluloid objects show the same finding: camphor content declines, mass decreases, and the material becomes brittle, shrinks, and tends to crack (Elsässer et al. 2021; Elsässer et al. 2023). After several decades, or more than a century of natural aging, residual camphor often settles around about 15 percent (cf. Elsässer et al. 2023). The loss of plasticizer accounts for the change in dimensions: the material sheds substance and contracts (King, Grau-Bové, and Curran 2020).

That is precisely why Blue Amberols and other celluloid cylinders should not be cleaned with alcohol. Camphor is highly soluble in ethyl alcohol (Reilly 1991). Alcohol therefore draws out the plasticizer and accelerates the embrittlement that has long been under way. The familiar smell of camphor that rises during reaming or improper cleaning is very real; it is the plasticizer itself, now volatilized.

The Plaster Core as Scapegoat

That celluloid shrinks even without any core at all is unmistakable with older Lambert cylinders. Many of these consist entirely of celluloid. Through the loss of camphor they become smaller in both length and diameter, often fit only partly on the mandrel, have to be worked down carefully, and still tend to skip during playback. A swelling plaster core cannot be the root cause, because none is present.

The physics of building materials likewise argues against the picture of a core that swells continously for decades. Hardened plaster is only weakly hygroscopic. Equilibrium moisture in the range of bound moisture, at room temperature, is on the order of a few percent by mass (cf. Adamski, Adamska, and Pakowski 2018). Liquid water can be taken up by plaster to a considerable degree and in the process reduce its strength (cf. Adamski, Adamska, and Pakowski 2018). It does not follow, however, that a sealed core, fully cured since manufacture, swells year after year under ordinary indoor air so strongly that it bursts the celluloid sleeve.

A practical finding from my own circle of friends fits this picture. A collector whose house suffered water damage reported that several Blue Amberols stood fully submerged for hours and, after drying, still fitted the mandrel without any problems. If lasting swelling of the plaster were the decisive mechanism, this extreme case ought to have rendered the inner form unusable.

Tension Instead of Swelling

The overall picture is thereby reversed. The plaster of Paris core does not swell and does not burst the celluloid. The celluloid loses camphor, sheds mass, and wants to shrink. The rigid core will not allow that. A steadily rising hoop stress develops instead. Fine cracks appear first at the ends, where the celluloid sleeve is thinner. Under unfavorable conditions, a cold shock, for instance, or rough handling, that sleeve may split lengthwise. The cylinder is then, a total loss. In other cases the plaster of Paris itself can develop cracks, or pieces of it begin to crumble.

The reamer therefore remains a useful tool, but under a different aspect. The inner form grows tighter because the shrinking sleeve compresses the plaster, not because the plaster soaks up like a sponge. Anyone who works the interior is treating a symptom of celluloid aging; they are not removing the cause.

The problem in general affects all celluloid cylinders. In coreless types it appears as free shrinkage. In the Blue Amberol it becomes more dangerous because the core blocks the movement and stores the stress in the sleeve until the material gives way. A similar fate affects the later Lambert cylinders with papier-mâché cores.

What Remains?

The swelling myth survives because it is vivid and because even major reference texts keep passing it on. I, too, accepted it for far too long. Materials science, however, tells a less comfortable story: Blue Amberols age from the outside in through the slow loss of their plasticizer. To store them dry, cool, without alcohol, and without sudden temperature swings is to steal time from that process. The plaster core is less the culprit than the unyielding form against which the shrinking celluloid destroys itself.

Selected Sources

• Adamski, R., Adamska, A., and Pakowski, Z. “Identification of Moisture Transport Mechanism in Gypsum during Convective Drying.” In IDS 2018. 21st International Drying Symposium Proceedings, 379–386. Valencia: Editorial Universitat Politècnica de València, 2018.
• Elsässer, C., Micheluz, A., Pamplona, M., Kavda, S., and Montag, P. “Selection of Thermal, Spectroscopic, Spectrometric, and Chromatographic Methods for Characterizing Historical Celluloid.” Journal of Applied Polymer Science 138, no. 21 (2021): Art. 50477.
• Elsässer, C., Angelin, E. M., Montag, P., Hilbig, H., Grosse, C. U., and Pamplona, M. “Frozen, Cold, or Cool? Chemical Assessment of the Effectiveness of Storage Conditions for Celluloid 3D Objects.” Polymers 15, no. 20 (2023): Art. 4056.
• King, R., Grau-Bové, J., and Curran, K. “Plasticiser loss in heritage collections: its prevalence, cause, effect, and methods for analysis.” Heritage Science 8 (2020): 123.
• Reilly, Julie A. “Celluloid Objects: Their Chemistry and Preservation.” Journal of the American Institute for Conservation 30, no. 2 (1991): 145–162.
• Selwitz, Charles. Cellulose Nitrate in Conservation. Getty Conservation Institute, 1988.
• “Blue Amberol Records.” Wikipedia. https://en.wikipedia.org/wiki/Blue_Amberol_Records

Translated from the German original “Blue Amberols: Ein Mythos, der einfach nicht verschwinden will.”
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