Using a solution of NaOH will not result in an evenly etched surface unless the glass is constantly agitated. It will be difficult to control the amount of etching and it will take a long time. Also NaOH results in a 'milky' frosting -- the original idea was to have a 'clear' screen for photographing stars.
HF vapour in a closed container is much better. But, as mentioned before, one side must be protected with a layer of wax. Do this in the open air or in a chemistry laboratory fume cupboard. Stick the protected side of the glass to the lid of a large polythene jar. Put a small amount of HF in the jar, screw on the lid. Take it off, after the period determined by previous experimentation, and wash under the tap. You can get an even, fine, etch this way. Keep HF away from skin and clothes and don't breathe the vapour. If you can see the 'frosting' it's already too much. By the way the glass must be very clean before you start. Clean it in Chromic Acid or Aqua Regia (=H2NO3+HCl). To carry out this slightly hazardous operation (the Hydrofluoric Acid thing) you need chemistry laboratory facilities and so getting hold of chromic acid should not present a problem. On the other hand you could keep watching eBay for the right screen. That's what I'd do. Don _______________ Dr E D F Williams http://personal.inet.fi/cool/don.williams Author's Web Site and Photo Gallery See New Pages 'The Cement Company from HELL!' Updated: August 15, 2003 "Oh my God! They've killed Teddy!" ----- Original Message ----- From: "mike.wilson" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Thursday, November 20, 2003 5:49 PM Subject: Re: Making your own (WAS Re: Anybody know where to get an SD-11 focusing screen?) > Hi, > > Fred wrote: > > > > If you have a piece of glass of the right shape and thickness, it > > > would be easy to "matte" one side with a solution of sodium > > > hydroxide. Some experimentation would be needed to get > > > concentration and times. > > > > Gee, I don't think that NaOH will do anything to glass. I think > > that, to etch glass, you need hydofluoric acid (HF). > > Needs to be quite concentrated and to stay there for a few hours but it > will make it nice and frosty with about 1/10th the danger of HF. > > mike >
