[alt-photo] Re: repairing scratch in the glass over the vacuum frame

[Johnny Brian]

Lead makes glass yellow. Lead (as in leaded crystal) was added so ordinary blue-green soda-lime glass would appear colorless. Glass with significant lead content (to block x-rays) is perceptibly yellow. I think float glass today is made on molten tin. 
Johnny Brian
On Jan 29, 2012, at 2:02 PM, Jacques Augustowski wrote:

> 
> Look at the glass sideways if it is green the more lead it has, all
> architectural glass is green. The bluish color in more expensive glass
> means that it has less lead and it is said that it is best for
> transmitting UV rays. Float glass has both surfaces perfectly parallel
> ,  that means that this glass is optically good.  They call them
> float glass because when the glass pane comes out of the furnace in a
> molten state it cools down on the surface of molten lead. The green
> glass also attenuates more IR rays than the bluer glass. 
> Jacques Augustowski 
>             PY1HY 
> On Dom 29/01/12 15:36 , Nelson Mark ender100 at aol.com sent:
> I used 1/4 " plate glass and had the corners rounded and all the
> edges ground with a slight rounding.
> On Jan 29, 2012, at 10:52 AM, Richard Knoppow wrote:
>> 
>> ----- Original Message ----- From: "Earl Johnson" 
>> To: "The alternative photographic processes mailing list" 
>> Sent: Saturday, January 28, 2012 9:58 PM
>> Subject: [alt-photo] Re: repairing scratch in the glass over the
> vacuum frame
>> 
>> 
>>> I have replaced the glass on both sides of my flip top Nuarc with
> Ace Hardware window glass with no apparent decrease in exposure times.
> On my unit, the glass is held in place by screws that are easily
> removed with the correct Allen wrench (hex key).
>>> 
>>> Earl Johnson
>> 
>>   My only experience with this was replacing the missing glass top
> on a military contact printer (forgot the name) which uses a group of
> argon lamps as the source. I made a crude measurement of the UV
> transmission by looking at the fluorescense of the brightening dye
> used in paper. I used thin plate glass, it was obvious that the glass
> attenuated the UV significantly. The printer also has a diffusion
> glass, the original was cracked but servicable after repairing with
> optical cement. That has no noticable UV attenuation. The glass I got
> for the top is green when looking at the edges, the diffusion glass is
> clear. I did not attempt to find glass with good UV transmission
> although I think such glass is available for special uses such as
> green houses. The UV transmission is sufficient for normal
> silver-gelatin contact paper. There were no bits of the original left
> so I don't know what its characteristics are. I doubt if this printer
> is intense enough for use in any POP alterna
>  tive printing process.
>> 
>> 
>> --
>> Richard Knoppow
>> Los Angeles
>> WB6KBL
>> dickburk at ix.netcom.com [2] 
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