Why does imitation ivory turn yellow?

The formation of a yellow color over time is a natural consequence of the materials used in the manufacture of some imitation ivories. First some background.

There are two ways to make a white material. The first is to use a high molecular weight organic material which is so large that it reflects all the light in the visible spectrum. This is the approach used with celluloid, phenolic plastics, nylon and Teflon. The second approach is to utilize crystalline particles that are large enough to reflect all the light. This is why filled polymers look white.

The most common form of yellowing is oxidation. If there are reactive functions or sites in the material used, these sites can react with oxygen from the atmosphere. Generally the worst offenders are the phenolic resins (used between 1930 and 1980) which are generally not completely crosslinked during manufacture and may, therefore, retain some reactive groups. After some years, oxygen molecules permeate the outer layers of the material and react to form highly colored quinones and hydroquinones - hence the yellow/orange color. There is no coating that can stop this process. It also does not seem possible to chemically reduce these species back to their original form without damaging the polymer.

I've tried to remove the oxidation using polishing creams such as Flitz or MAAS, but the best that I could do was to lighten the color on the exposed facts. The concave portions and grooves were impossilbe to reach with enough "force" to abrade the surface. Another product that I haven't tried is Autosol, but it seems similar to the others.

One word of warning: NEVER get bleach anywhere near the imitation ivory of a bagpipe.  It will not whiten the surface.  Bleach is an oxidizer and will turn a slightly yellowed material to an ugly orange-brown in a matter of moments. Peroxide will do the same thing. To remove the yellow, the goal would be to chemically reduce the offending species and this does not seem possible.

Some plastics don't show yellow color changes:

• Celluloid (a.k.a. pyroxylin, Parkesine, Xylonite) contains no groups that can be oxidized , so it is unaffected by oxygen.
• The plastic in filled polymers may still turn yellow, but these materials contain so much of a non-yellowing filler that there is only a very thin layer of yellow on the surface above the white filler. Consequently, the yellow polymer isn't really thick enough to affect the color of the reflected light and the piece will appear to be white.

You can learn about these and other forms of plastic here.

I'd be happy to discuss the yellowing of imitation ivories further if you would contact me.