What makes traditional lead crystal different to mass-produced glass & how to identify it

Written by Chris Blade MA (RCA), CEO of Cumbria Crystal

Raw materials melting in a furnace at 2400 degrees centigrade

 

As few people have detailed knowledge about hand-made 24% lead-crystal and what differentiates it from mass-produced products, I hope this information may be of interest and increase understanding and appreciation of Cumbria Crystal's products.

Lead crystal, invented in England in the 1680s by George Ravenscroft, revolutionised the world glass industry. The addition of lead oxide to glass (in our case 24% - the minimum requirement to be legally called 'crystal') changed the global glass industry. It helped make glass clearer, melt at lower temperatures and add weight to the material. This weight alters the refractive index of the glass meaning it can bend light more than other glasses, hence it sparkles more! In fact, the high specific gravity of cut crystal is essential to maintain this quality.

Due to the popularity & prestige of crystal in the 18th Century the English government imposed high taxes on crystal leading to design evolutions & bizarre taxes such as the window tax which led people aspiring to show wealth to paint windows on their houses where there were no windows.

The advantage of lead-crystal over soda-lime glass, or today’s commonly used barium based glass, is that the composition of the material makes it eminently suitable for hand cutting designs, using diamond & sandstone wheels, on a lathe. It is also especially good for polishing using concentrated acids, to make cut-crystal - a process in which we are one, of only two companies I know of, in the UK that can still do it.

However, the use of lead oxide to create real crystal, sometimes referred to a flint glass has several production, health and financial disadvantages. Sadly, these have led nearly all modern manufacturers to abandon it.

1. It is massively expensive to make the raw materials for lead crystal compared to modern glasses.
2. The safety of our glassblowing team & employees. In its raw, powdered state lead oxide can be absorbed through the skin, inhaled, or ingested. This can be dangerous to health, so we go to great lengths to protect our staff through safe working practices, regular health monitoring and blood tests, plus the use of Personal Protective Equipment (PPE) for the few processes where there is risk of exposure. This all happens before the glass is melted. During the melting process the material is chemically bonded with the other glass ingredients and no longer presents a hazard. It should be noted that once the crystal is made there is no longer a risk to health and it is safe for staff, and of course, customers to use.
3. It is far more difficult to blow than modern materials, thereby reducing production yields. 
4. It has a long working life meaning it cannot be worked as quickly as modern materials as it stays liquid longer.
5. It takes two days to melt the glass from the raw material before it is useable - modern glasses are often melted overnight. 
6. It is prone to lead evaporation from the surface of the glass which slightly alters the refractive index of each piece resulting in an effect known as cord.

Cumbria Crystal’s USP is that we aspire to produce the best handmade lead crystal in the world exclusively using traditional skills, materials and heritage processes. Whilst emulating historic processes as closely as possible we achieve a quality of glass far in excess of those achieved by Masters of the past. Crystal made this way, by its very nature, has defining characteristics which, if one knows what to look for, identify the crystal beyond doubt as genuine, completely made by hand from traditional small furnaces using heritage processes.

Glassblower, Andrew Round, preparing the raw material (batch)

Today, the few large companies that still produce crystal melt it in large factories in extremely big, modern, continuous-melt tank furnaces, as opposed to hand-made closed-pot clay crucibles. Modern furnaces melt between 10 - 30 tonnes of glass each day compared to Cumbria Crystal's one tonne per week. The quality of glass produced (even if they were to use the same raw materials as us) is different. Modern furnaces continually add raw materials in one end (imagine filling a swimming pool with a hose) and take out glass continuously from the other. This cannot stop, day or night, 7 days a week otherwise the furnace would overflow! This is useful for large-scale production, but it is definitely no longer historically or artistically accurate.

The advantage continuous melt furnaces have is that with such a volume of glass is moving through the furnace at any one time, the slow continuous movement of glass helps eliminate small bubbles and dissolve cord. Like stirring sugar into a cup of tea.

Other contemporary approaches to making glass completely free of bubbles and cord (although not usually lead crystal) is to melt the raw materials in platinum crucibles, and to stir the mixture slowly until it is used use. Glass does not adhere to platinum and the metal does not melt so it is perfectly suited. This approach enables the production of stunning optical quality glass, but at a mind-boggling cost. Even smallish crucibles can be upwards of $2M and stirring blades $500,000 each. This process is therefore reserved for specialist items such as camera lenses.

The melting process at the beginning

Bubbles, cord and stone (within reason) are defining characteristics of traditional glass melted in crucibles so expected to be visible to some degree. They are also the source of most questions about lead crystal as no glass made using traditional processes is free of these, especially since the glass cannot move in this type of furnace. If one were to study, even the most expensive antique English crystal, from the hay-day of English crystal making in the 17th, 18th, 19th Centuries these characteristics are apparent.

Cumbria Crystal goes to great lengths to minimise this; so much so that the unique characteristics of lead crystal are sometimes mistaken for flaws by our clients rather than recognising that they visually validate the authentic integrity of the historic processes. As most people’s experience of glass is limited to modern mass-produced glass (as discussed above) made in hundreds of thousands, or millions by automated processes, this is hardly surprising. It is one of the reasons Cumbria Crystal stubbornly strives to keep the crafts alive in the UK & to share the knowledge of what to look for in genuine traditional crystal.

Bubbles | Traditional glass is melted in handmade, clay, closed pot furnaces. The molten glass stays stationary in the pot until it is used. Imagine filling a bucket with water and taking it out one cup at a time. At the end of the week unused crystal is ladled out, fritted in cold water and then recycled with new raw materials the following week. This helps to ensure the best glass possible but is energy intensive as it involves remelting materials.

To minimise the likelihood of micro bubbles, glass is melted very slowly over a two-day period. Most factories melt overnight. By extending the melting time it is hoped that more of the micro bubbles reach the surface. Even at temperatures of 1450 centigrade the glass is still quite viscous so not all bubbles reach the surface. Traditional manufacturers float specialised ceramic rings on the surface of the glass and gather the glass from inside the centre of these. Again, this helps minimise the number of bubbles gathered on a blowing iron, but at the expense of slowly degrading in the extreme environment. These rings slowly degrade, introducing extremely minute flakes of ceramic into the glass – these are called stone.

Stone | Usually observed as microscopic ceramic elements that flake off the rings, or the ceramic crucible itself (pot) due to the corrosive action of liquid glass and heat on the clay. Stone can look like tiny white, or black, flecks floating in the glass. Pots generally last 3-6 months before requiring replacement. Even with a new pot, stone cannot be fully eliminated. It is an inherent in all glass melted in traditional furnaces.

Cord | Cord is a thin, but visible, difference in the refractive index of the glass. It causes light to refract inside the glass in a wispy manner. This is most commonly visible in the thickest sections of a crystal product, such as the base. Cord is one of the defining characteristics of lead crystal since and is inherent in traditional lead crystal glass. Cumbria Crystal works with 24% lead crystal as opposed to 30%+ (called ‘full-lead’ crystal) to minimise the effect as the higher the lead content the more visible it generally is. However, cord is not unique to lead-crystal and does appear in other types of glass, depending on the way in which it is melted and type of furnace it is melted in.

An example of cord in the base of a glass, which has been intensified so it is easier to see

The effect is primarily caused by evaporation of lead & other materials from the surface of the liquid glass as it sits in the crucible waiting to be used. Whilst evaporation is minimal it alters the chemical composition of the glass, thereby microscopically altering the refractive index of the material on the surface of the pot. As a glassblower gathers liquid crystal, by dipping a blowing iron into the glass and rotating it, the surface crystal mixes with the crystal under the surface creating a slight variation in the composition of the glass. This happens throughout the day and evening, but is more common after a short break or towards the end of a working day. If the glass is able to ‘move’ whilst liquid this dilutes and homogenises the glass minimising the effect making it less visible. On cooling, changes in refractive index become apparent. In areas where the glass needs to remain thick ie. bases of glasses or decanters, the glass does not move as much as the sides of the vessel, and therefore the effect is more visible.

To try and minimise this our glassblowers rake the surface of the glass between every gather. This has a positive benefit, but cannot eliminate it, but significantly reduces the number of products that can be made each day. We do it, despite this. In the industry it was known as “raking away the profit”.

For information: Evaporated lead is extracted with exhaust gasses from the furnace and filtered to remove lead before reaching the atmosphere. The process is expensive, but it ensures the company emits less than 1% of allowable emissions as prescribed by our Environmental Permit.