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HISTORY OF THE SPECTACLES
We all have a tendency to take for granted the lenses we use, regardless of the material from which they are made. Compared to lenses of old that contained bubbles, stria and generally uneven surfaces, the quality we expect today is much different. Today have such excellent quality that we seldom think about those defects during the course of daily business. Let’s look at the process, starting with glass.
GLASS : A simple definition of glass is fused mixture of silica, usually in the form of natural sand, and two or more component glasses, such as soda, lime, or potash. There are single component glasses, such as sodium silicates. Glass is generally transparent or translucent and is brittle and when heated becomes soft and ductile finally melting.
An important concept is that the glass is not crystalline and when cooled to room temperature may seem hard, but in fact, has an extremely high viscosity and is actually still liquid.
It is not certain the exact date that glass was discovered. In 1857 Pliny wrote the following : “ The story is, that a ship, laden with nitre, was moored upon (the river Belus). The merchants, while preparing their repast upon the seashore ; finding no stones at hand for supporting their caldrons, employed for the purpose some lumps of nitre which they had taken from the vessel. Upon its being subjected to the action of fire in combination with sand of the seashore, they beheld transparent streams flowing forth of a liquid hitherto unknown. This, it is said, was the origin of glass.” This supposedly took place in Phoenice in Syria.
According to another historian, the earliest glaze known is that on stone beads during the Badarian age in Egypt, about 12,000 BC. This glass was green in color and was used as a basis for making small figures.
The use of aids for improving sight were another legend. Spectacles were worn at the time of Confucius in 500 BC. There are even claims that a Chinese Emperor used crude forms of lenses in 2283 BC. The lenses were made of amethyst, quartz and topaz.
In 1927, E.J. Fordyke found some crystal magnifiers in a tomb in Crete, which were dated from 1600 to1200 BC. Although spectacles are not thought to have existed prior to the 13th century , S.E. Hieronymous had a figure of lion, a skull and a pair of spectacles on his crest between about 32 to 420 AD and is generally considered the “Patron Saint” of spectacles.
The more one reads about the origin of glass and spectacles the more confusing it becomes due to the disagreement of historians on who and when it was discovered. We’ll leave the controversy to the history buffs and look to the production of the Crown glass of today.
Early glass was fraught with defects due to the problems in controlling the process of producing lenses. In fact rock quartz crystal, used for early lenses (C1700) continued to be used. Quality lenses from quartz better than the glass of that time.
The production of glass isn’t easy. Silicate sand is heated to between 2400 and 2600 degrees Fahrenheit. That together with other additives, such as soda ash , become molten and flows down this trough where it is snipped off in specific lengths.
Each length that’s snipped is a specific gram weight dependent upon the mold blank size it is to become.
Each piece drops into a mold and the red hot glass bank is die pressed by a hydraulic machine to form the appropriate shape required.
The glass blank must cool slowly. This is known as annealing. This is necessary to remove the stress in the blank from the molding process.
Since the glass blank is unfinished on both sides, it must have at least one of its surfaces finished at the factory before being released as a semi-finished blank, or have two surfaces finished if it is to be a finished blank.
HARD RESIN: In 1945 a large tank car set on a spur rail outside of Pittsburgh Plate Glass Company (PPG) in Pittsburgh Pennsylvania. In it was a plastic monomer that had been used in the development of aircraft canopies. Someone had the idea of attempting to produce a lenses from the material and a new product was born. The material in that tank car was Colombian Resin batch #39 or CR-39.
Casting is the process of mixing an initiator into a monomer to start the process of cross linking the molecular structure, commonly called curing. Once completely cured the plastic becomes a crystalline structure and cannot change shape again. Since this can take place inside a mold, a replica of the form inside of that mold is created. Lenses are produced in this manner.
The cured lenses are then removed from the molds and inspected for cosmetic acceptance. The new lens is also now ready for mechanical inspection. The lens meter or Humphrey lens analyzer is typically used utilized to verify sphere and cylinder power of the lens. Center thickness is then checked against specifications.
At this point, if the lens is to remain uncoated, it moves on to packaging if, however, the lens is to be coated, it follows a different path.
There are many method of coating a lens but the most common method is the “dip coating’ process where a batch the lenses are dipped in to the coating and then cured either thermally or through the use of ultraviolet.
Since abrasion resistance is an extremely important point, certain tests are conducted to verify that each batch of coated lenses maintains the same high standards of abrasion resistance. Other tests verify that the coating will remain intact during various changes in the environment that include temperature and humidity.
Packaging is next followed by stocking the finished product into the warehouse or distribution center. There are over 8,000 different types of lenses in the typical warehouse. when an order is received the stock person picks each lens individually and packages it into that customer’s box for shipment that day.
AFTER MANUFACTURING :
Lots of things happen after the lens leaves the plant, most important of which is laboratory processing.
In the laboratory, the lens undergoes layout for surfacing, surfacing layout for edging, edging, tinting, and coating. There is far more involved in laboratory processing than we could cover in the space remaining. It should be pointed out however, that a good laboratory is very much involved in assuring good quality to the customer.
Another extremely important area for the laboratory is good coating techniques on the back surface of the lens.
A coating system first involves the cleaning of the lens.
Once the lens has been completely cleaned and dried, it then is coated using the same technique. The lens is dropped down over a fountain of coating material and spun at a specific RPM to achieve the proper thickness. From there the coating is cured. The finished lens is then edged and inserted into a frame.
This process to bring the end customer a quality prescription is improving every day. It’s fascinating that technology has done and continues to do in the ever improving science of “the lens”
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