1920s vintage red thermoplastic potting compound. What is it made of?
I am a retired amateur historian of communications technology. Disposable dry batteries of the 1920s were capped with a red or brown thermoplastic compound generally referred to as 'sealing wax'. It appears to consist primarily of tree resin and finely pulverized brick dust. It flows like room temperature honey at about 120 C. After pouring and cooling the surface can be re-heated with an infrared lamp enough to be embossed with a maker logo and various text. It is rock hard, brittle and will not exhibit cold flow for decades at temperatures below 30 or 40 C. These batteries are extremely rare today and I have been making museum grade replicas as a hobby for over 5 years. To my knowledge, I am the only person that has taken on the task of recreating these batteries to museum standards.
I have failed to find on-line any vintage references to the actual formulations of this thermoplastic compound. I have found no commercial names for the material . The material I have been using as a substitute is called 'red pitch'. It is used by gold and silversmiths to back malleable metal when hammering relief features. It gives excellent looking results BUT over time (several months) it shows cold flow problems at ordinary room temperatures if the poured surface is stored in a vertical plane. (Many of these batteries are installed laying on a side.)
Questions that come to mind are:
- What kinds of vintage publications might have information on such formulations? It was used in many hundreds of millions of dry batteries made from about 1915 to 1940. (It is not the asphalt based compound used on automobile batteries.)
- If I were to contract for an analysis of this material. What type of service should I be looking for? (I'm on the budget of a retiree.)
- I can only supply a few cubic inches of the vintage material. Is it reasonable to think that an analysis could be performed on such a sample size?
Any comments welcome.
Thanks for your time,
Robert Lozier – KD4HSH
Monroe, NC, USA