April/May 2001 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - April/May 2001 Issue

The Telescope Nut
by Jeff Baldwin

Making Glass Shine

Glen Youman and I have been having discussions about polishing mirrors for years. According to Jean Texereau, rouge, cerium oxide, and other polishing agents heat the glass on the top few thousand molecules to the point where they smear, thus making a shiny polish. His proof for this was that etching and examination with high-powered microscopes would show particles of polishing material under the glass surface, indicating that the glass 'flowed' over them during the polishing process. This makes sense, and we have been flying with it for the past six to eight decades.

Glen disagreed with Texereau and explained his belief to me. He said that if Texereau were correct, as we moved glass in a state of melt it would be able to be pushed around. When we look at the mirror in the knife-edge test or the Ronchi screen, we would have low spots where we removed and pushed glass away, but also high spots where it would pile up and deposit. We would also have clean polishing agent since the glass wasn't coming off itself, just being moved around. However, we don't see the polishing agent remain the same; we see white added to it, the tell-tale sign of glass being added to the mix. We also don't see glass piling up when we push it from one area to the other and examine it with the knife-edge or Ronchi.

If Glen is right, Texereau must be wrong. Glen's theory, at least as I remember it, is that a shearing action must be taking place to move the glass while we are polishing it.

Months go by, and finally Glen shows up with a catalog from Salem Distributing, "Precision Electro-optical". The book explains their spin (most likely the truth) on how polishing occurs. It shows it as being a chemo-mechanical reaction, where the glass is not only sheared but also removed by a chemical reaction between the glass and the cerium oxide to form cerium silicate. Specific temperature and pH concentration of the cerium oxide slurry can maximize the efficiency of the polishing action.

According to Salem Distributing Company:

"...Cerium oxide is the most abundant of the rare earth compounds. It has the unique ability to polish glass by a combination of mechanical and chemical action. Particles contacting the glass react to form a thin layer of cerium silicate. Unreacted cerium oxide quickly wears away the cerium silicate layer yielding brightness and speed exceeding any other glass polishing compound."

(According to Salem all other polishing compounds function only through mechanical action; these include Aluminum oxide, Zirconium oxide, rouge, colloidal silica, etc.)

I guess my next question would be how are other materials brought to a polish? Ceramics, metals, and other materials are also polished with the same agents, but lack the silicon that glass has. The question after that would be how does rouge, which is iron based, polish glass? And possibly this has something to do with why rouge is so slow. Maybe it is strictly a shearing action rather than a chemo-mechanical action.

Back to the cerium oxide story. My favorite agent, Barnesite, is 95% cerium oxide. Glen saw on the ATM Internet list a guy who avoided Barnesite because he had heard that Barnesite was radioactive and he would stay with the non-radioactive cerium oxide. The radioactive question came up to us not too long ago, so I brought a Geiger counter home from work one day and tried to measure a few times around the house. My smoke detector was difficult to read, but it did have a few hits above ambient background levels. My Barnesite did in fact make the counter pop more actively than holding the counter in clean air, but not much. It is a very mild alpha emitter, so eating it and breathing it is not advisable. When we use it we have it mixed in a slurry. Combined with this, we are exposed to it only once a month or so, and for short periods. If it were hotter (nuclearly) I would be concerned, but I'm certain watching TV is far more dangerous. What could happen, hair loss? (for those who don't know me, I'm bald). Seriously, we deem it safe enough to use at the rate we use it and with the method we use.

There is another non-cerium polishing agent, zirconium oxide. I have never used it, seen it, owned any, and have no idea how it works (mechanical action only and it is much slower than CeO) or if it is any good or not. If anybody out there has used it, please let me know.

The great thing that separates science from dogmatic thinking is that it's OK if the story changes when evidence indicates we were wrong or that a detail was wrong. Sometimes our egos won't let us make the change, and at those moments we are not scientists.

I plead guilty as charged, yet I'm the first one to point the finger when it's somebody else. Anyway, here is one of those moments. I've been running with the melting story all my ATM life, and I now realize that it was most likely a quick and effortless explanation of polishing, yet incorrect.

Clear Skies!...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.