Video of Glass Production
Both lead crystal glass and full lead crystal glass are made from a mixture of sand, potash and lead oxide. There is a British specification (BS 3828: 1964) for each type. Lead crystal glass contains a minimum lead oxide content of 24% and is manufactured primarily for it’s beauty of appearance, particularly when decorated by a skilled glass cutter. Full lead crystal glass contains an even higher proportion of lead oxide, in fact, not less than 30% which greatly enhances this beauty although it is more costly to produce.
The Glass House
The scene presented by the glass house while the work of glass making is in progress is full of interest and fascination. Men are engaged in groups around the furnace where the glass is melted, performing in the glow from the crucibles, the complicated, masterly, and often graceful movements of their craft. Each group is known as a “chair” and consists of 4 or 5 persons: The “workman”, who is the principal, two who are known as “footmaker” and “servitor” – and one or two boys who are employed in performing minor operations, and carrying finished articles away. A certain part of the work is apportioned to each member of the group, according to the design of the article on which the “chair” is engaged.
To begin, a ball of molten glass is gathered from the pot on the end of the blow-iron, which is a steel tube about 4 ft. long. It is continuously rotated in order to distribute the glass evenly. The gathering of glass is next rolled on a polished iron table, called a “marver” to smooth and regularise the surface. The glassworker now blows through the tube and the hot glass becomes a hollow sphere, the blow-iron being turned, tilted and swung in a way which almost magically influences the shape which is developing. The glass is further shaped as the iron is rolled up and down on the long arms of the glassmakers chair. A few very simple tools are used – such as steel pincers, callipers, shears, wooden measuring sticks and clappers. The stem and foot is now added, and the object is transferred for further working to a pontil, or steel rod, to the end of which it is attached by a blob of hot glass. Re-melting is necessary before superfluous glass is sheared away from the rim and the final touch is given. The still glowing object is cracked off the rod, dropped into a carrying apparatus and transported swiftly to an annealing oven or “lehr”
The lehr or annealing chamber is a long tunnel down which the glass is conveyed slowly, passing gradually from a high temperature to the coolness of the ordinary atmosphere. This is necessary because, if the glass were allowed to cool rapidly, it would probably shatter owing to the strains and stresses within it. On removal from the lehr, each article undergoes a searching examination for flaws. At this stage a scientific instrument is bought into play which reveals by means of polarized light, any trace of imperfect annealing.
This is done by holding the glass against the edge of a revolving wheel.
It sounds a very simple process, but actually it calls for a very high degree of skill.
The glass must be cunningly guided to produce some of the lovely and intricate effects that are possible.There is an infinite number of patterns and combinations.Some of these have curious and interesting names such as “diamonds”, “laced diamonds”, “hobnails”, “splits”, “fans”, “stars”, “flutes” and “jewels”.
The pattern,which has been indicated on the glass “blank” by specially trained markers,is first roughly cut by the carborundum wheel.The glass is now ready for polishing.
Polishing is done by immersing the glass in a vat containing a mixture of hydrofluoric and sulphuric acid.About a minute’s contact with these corrosive chemicals and a thorough rinse afterwards transform glasses with matt decoration into sparkling pieces of crystal.
Characteristics of Crystal Glass
It is quite normal for people to wonder why small irregularities cannot be eliminated entirely form hand-made crystal glass.
The answer is just that very fact – that it is hand-made.It is impossible to eliminate small variations.They should not be regarded as defects.Glass is one of the most difficult materials with which to work.
A Seed or Bubble
During the fusing and melting of the raw ingredients which form lead Crystal,gases are evolved and air between the particles is entrapped.During the refining process nearly all these seeds or bubbles are removed.
Small isolated ones sometimes remain and should not be regarded as flaws.
When two peices of crystal are joined together during the making of an article a small air bubble may be encased.Wherever possible,this inclusion is avoided,but in some cases it must be regarded as inherent in the product.
Strange as it may sound,slight variations in dimensions are the hallmark of fine hand craftmanship.Absolute uniformity can be achieved only in assembly-line products and it is not possible or even desirable in quality ware,which depends so much on the skill and artistry of individual craftsmen.
When glass is perfectly homogeneous it does not contain cords.However,in practice,this state of absolute perfection is rarely achieved and slight differences in refractive index occur which are visable to the naked eye.Unless this effect is very pronounced it should not be regarded as a defect.
Mould and Tool Marks
These are sometimes apparent on the surface of the article and,if slight,should not be looked upon as imperfections.
The artisan frequently snips off excess glass when shaping the piece whilst molten.
Slight marks from the shears can frequently be seen around the edge of hand-made articles and at the end of of the handles of jugs.
These marks are a normal characteristic of the glass and are not flaws.