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The story of glass and crystal polishing

Cumbria Crystal uses hand, acid and fire polishing in the making of its crystal. The company’s unique selling point is that it is dedicated to the creation of the best crystal that can be made exclusively by hand, using traditional 2000 year-old glass making processes. Therefore, it is little wonder it takes an average of 12 days to create each and every piece of crystal.

Below takes you on a small journey of lead crystal and why polishing is such a fundamental part of what we create.

For millenia the holy grail for glass makers was clear, colourless glass. The discovery of English lead crystal, subsequently called flint glass or white glass, by George Ravenscroft at the Savoy glass furnace in London led to huge improvements in the quality of glasses available.

 

Prior to the patenting of English lead crystal in 1674 the world centre of glassmaking was the Italian city of Murano. Despite the use of the best ingredients Italian glasses had colour casts caused by impurities in the raw materials. To counter this the trend with Venetian glass was to blow vessels incredibly thin, effectively diluting the colour and making the glass appear clear.

The discovery of lead crystal dramatically shifted the sands and led to the rise of the English lead crystal industry & development of new styles of glass that would dominate global crystal production for the next 300+ years.

The introduction of lead oxide into the glass recipe brought significant benefits.

Most importantly ‘clear’ glass could be produced without needing to be thin; lead crystal, with a high specific gravity refracts light better than other glass – in practical terms this means that when cut with the distinctive patterns we associate with lead crystal, it sparkles far more

It could be blown heavier, whilst remaining clear, leading to the evolution of traditional baluster glasses with beautiful heft.

Crystal possesses a longer working life than other glasses as it stays malleable for longer making it easier for the glassblower to work and it could also be melted and worked at lower temperatures reducing the amount of energy required. Being slightly softer than other glasses, it is uniquely suited to the application of decorative cutting with abrasive wheels; one of the most distinctive features that evolved from the development of lead crystal.

Finally, it is able to take a brilliant polish whether done by hand or through the use of acid.

Polishing glass is an art that requires huge patience, skill and nerve. It will generally be polished using at least one of the three ways describe below, by:

HAND

Hand-polishing uses processes, albeit with different materials, similar to those used for polishing wood, metal or stone. Consecutive grinding with progressively finer & finer abrasives until a smooth finish is achieved. Being a hard material, this means hand-polishing glass is a slow and delicate process with the ever-present risk of cracking if too much localised heat is generated through friction.

The abrasives used for grinding and hand-polishing glass include a diamonds, carborundum, aluminium oxide and sandstone. Once ground smooth a pre-polish is obtained by using a slurry of pumice power on a rotating cork wheel. All being well, the final polish is achieved with cerium oxide on a compressed felt wheel.

Achieving a full polish often requires 5-6 different stages. Discovering a single erroneous scratch requires the whole process to be repeated.

ACID

To polish the individual facets in cut crystal is not possible by hand. The distinctive sharp edges would be dulled during the abrasive processes and it is impossible to reach the finer sections of the design inner sections well enough to polish properly.

However, it is possible to achieve an extremely high polish in all the details by immersing lead crystal in a solution of 70% sulphuric and 30% hydrofluoric acid, at a temperature of 50C for 42 minutes. This process is exceptionally effective with lead crystal and is the reason the cut crystal industry was able to produce such decorative and ornate designs. Acid polishing, being incredibly dangerous and expensive to operate, is now only be done in a few specialist places in the UK and most companies shy away from it.

Hand-cut and acid-polished crystal is noticeably different to the imitation cut crystal produced through press or injection-moulding processes. Once you know what to look for the differences are obvious – sharp and precise cutting, with high refractivity, looks and feels very different to mass produced imitations.

FIRE

Finally, fire-polishing. Intense heat can sometimes be used to polish glass which has been partially ground. Cumbria Crystal uses this process to achieve a high polish to the delicate rims of drinking vessels which are prone to crack with the aggression of hand polishing. The rims, once ground to the correct height (or to remove a chip), are heated in an intense flame which causes the glass to partially melt and become shiny. Once complete the glass is brittle due to internal stresses caused by the heat. This is removed by a process called annealing which involves slowly heating the glass to 420C, soaking at that temperature to equalise the temperature throughout the piece, and then gently cooling it back to room temperature. This can be a very slow process as glass is a worse conductor of heat than wood!

Many mass-produced glasses often use an extreme variation to this process extreme heat to separate the glass vessel and create the rim simultaneously. Rims with a distinctive bulge at the top are very unlikely to be hand finished.

Fire polishing in inherently risky, as glass when heated or cooled too quickly, will crack through thermal shock. The temperatures required are very high as the glass needs to approach its melting point with the consequential risk of deforming.


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