Category:Process:Glass
From Conservation of photographic negatives
Contents |
[edit] Glass Base
[edit] historical background
1822 experiments by Joseph Nicéphore Niépce to use glass as a base material
1839 first achievement to use silver chloride without a binding media on glass by Sir John Herschel
glass plate negatives are in prior use between 1852 and the 1890ies
gelatin dry plates are used until the 1970ies
[edit] material
[edit] properties
substantially glass is composed of amorphous silicon dioxide, which builds the network (in more modern glass also oxides of boron, lead and phosphorus can work as network builders).
further components
- sodium or potassium oxides to lower the melting point of silicon dioxide (glass with an high content of sodium is called soda glass, with an high content of potassium potash glass, soda glass is much more stable than potash glass )
- stabilizers like calcium or magnesium oxides
- colors can be given by metallic oxides or dispersion of insoluble colloids
- matting can be reached by air bubbles or other imbedded compounds
During the 19th century mainly glass with a high content of soda or potash was used, which is not as stable as glass with high lime content. From the 1920ies on aluminum oxide was added in glass (aluminum and later also calcium silicates), which makes a much more stable glass then it was before.
[edit] production
flat glass: There are different techniques to produce flat glass: casting of the liquid glass into a flat model (sometimes traces of this model can be seen) is a quite old technique. Crown glass is made by rotating the glass (sometimes a curved line can be seen on the glass). In the quite common cylinder process glass is made to a cylinder which is then cut and bent open (elongated air bubbles can be seen in the glass). Glass can also be made to a cube cut into small glass plates.
These kinds of flat glass have a shiny polished surface and are mostly not thicker than 0.25’’. Glass plates can also be made of polished glass whereby the glass is grinded to a certain thickness, mostly more than 0.25’’ (this glass is flat without any waves in it and has a less shiny surface). Modern float glass is made by letting the liquid glass float on molten metal (perfect surface).
The edges of the glass plates additionally are treated especially with big formats to compensate unevenness from cutting. The bigger the plate the thicker it is. Typical glass plate thicknesses are (Kodak): 0.8; 1; 1.2; 1.5; 2; 3; 5 or 8mm.
[edit] use in photography
Initially glass for negatives is chosen by photographers with respect to its physical properties like color and surface. Later with gelatin dry plates also chemically high quality glass is preferred like Belgian glass, glass out of France and the US. Mirror glass is favored over window glass. Photo glass compared to window glass is made of white sulfate glass.
The surface of the glass must be treated thoroughly to reach as good emulsion adhesion as possible. Pre-treatment includes:
- cleaning with ammonium or potassium hydroxide
- polishing with a mixture of chalk and alcohol or water
- immediately before coating again cleaning to remove any dust
If old photographic plates are reused the old emulsion has to be removed by potassium cyanide, mixture of water and nitric acid or chromic with sulfuric acid, subsequent neutralization by potassium or sodium carbonate. But the re-usage of old plates, especially if they were intensified is not recommended since the old image can appear in the new emulsion maybe due to ultra microscopic silver particles in the glass.
Articles in category "Process:Glass"
There are 12 articles in this category.
ADG |
G cont.LNP |
P cont.TW |
