Looking into glass
From the pure white silica sands of Parengarenga Harbour to the bottles, jars and windows we come into contact with each day, the making of glass is both a science and an art. Louise Callan takes a look at this unique and timeless substance.
Twice a month, weather permitting, a tug and two barges set out from Auckland to plough their way up the eastern coast of the North Island to isolated Parengarenga Harbour, just south of North Cape.
The barges’ destination is the Kokota Spit at the harbour’s entrance where wandering sand dunes up to 40 metres high contain some of the largest deposits of high quality silica sand in the world.
Each trip about 2000 tonnes of fine white sand are loaded into the barges. The sand is brought aboard in slurry form by a submersible pump. The water runs back into the sea through scuppers along the side of the barge, leaving the sand behind.
Sea Tow, the company operating the barges, started working the sand deposits here in the 1930s. Today they are contracted by the two companies still making glass in New Zealand: Pilkington in Whangarei which makes flat glass and ACI in Auckland which makes bottles and glass containers. Each company has its own particular sand deposit to draw from. The locations differ because the chemical composition of the sand differs even within the same general area on the spit. Extraction is monitored by the companies and other agencies, such as the geology department of Auckland University, which follows the rates of extraction and the movement of the spit’s sand dunes.
Glass manufacture has a long although not always successful history in this country. To begin with, all glass products were imported but it was not long before emigrating craftsmen began their own workshops, sometimes combining imported materials like clear flat glass with further processing here. In 1871 the Weekly News remarked that the rapid success of the Union Street Glass Works in Auckland, in business then for a little over a year, was causing importers of wine glasses, kerosene lamps and lamp chimneys and jars to look for other kinds of merchandise to bring in to the country.
In recent years a number of companies and small operators, often working in specialised areas such as lampshades, lead-lighting and stained glass, have closed their doors. Pilkington acquired their Whangarei glassworks when McKendrick Glass Manufacturing Company, established in 1962, went into receivership shortly afterwards. Before that they had imported all the glass needed at their factory in the Hutt Valley to manufacture toughened safety glass.
Pilkington is as synonymous with glass manufacture as the American company Corning, makers of the first light bulb. It is a British company which started in 1826 as the small St Helens Crown Glass Company. It now has more than 60 subsidiary and associate companies in 18 countries. In 1959 Pilkington developed the float glass process, a development New Scientist magazine likened to one of much greater antiquity: “The Roman era was the golden age of glass technology… Their greatest achievement was a revolutionary new technology—glass blowing—which changed the face of the industry. Not until 1959, when Pilkington developed float glass—sheets of flawless glass made on a bed of molten tin—was the industry to undergo a revolution as dramatic.” At present, however, sheet glass is manufactured here by the Pittsburgh process, developed in 1914.
The Whangarei plant covers 3.25 hectares (8 acres) in the city’s industrial area. Sand is brought from the wharf by truck and unloaded into one of four storage bays. Each barge load is kept separate to avoid mixing sands of slightly different chemical composition. Although the sand is relatively clean it must still be washed, screened and processed in a large tank to remove small pieces of shell, grains which are too coarse and stone fragments. Brian Armstrong, the production manager, who, like a number of the staff, moved to this country after working for the company in Britain, says the Parengarenga sand is good compared with sources in the UK. “It has a better grain size, fewer contaminants and a helpfully low iron level.” This last is especially important when making sheet glass. Too much iron will give glass a greenish tinge unless extra chemicals are added to counteract it. The sand also has a very high silica content. Molten lava which forms the natural glass, obsidian, is usually about 75 per cent silica. The Parengarenga sand is 97 per cent silica.
Even with that high silica content it is not practical to make glass from sand alone. Silica has an incredibly high (1723°C) melting point. This is lowered and the molten glass made easier to work by adding other materials to the sand. The mixing room is where the glass making process begins.
The main raw materials—sand, dolomite, soda ash, limestone, sodium sulphate and felspar—are carefully measured, along with a little coal dust as this sand has no natural carbon content and carbon is thought to help the refining process. Apart from sand the only other local materials are dolomite and limestone. The rest come from Chile, the USA, Norway, Canada and Australia. Soda ash, which dramatically lowers the melting point, is the most expensive material, imported from the USA.
Batches of 1.4 tonnes are mixed, along with a little water to keep the dust down and to start some of the chemical reactions. The mixture is known as frit. The furnace devours 66 batches every 24 hours, 22 batches a shift. Even though the batches are so large, weighing accuracy is to within 200 grams. The exact chemical mix needed for each barge load of sand has already been established in the factory’s laboratory. A small amount of sand is dissolved in hydrofluoric acid and its elements analysed in an atomic absorption spectrophotometer, or AA for short.
The fit, mixed with cullet (broken glass), is now fed very slowly into an enormous furnace. The cullet comes from wastage and offcuts and is stored in a huge glitter-sharp mound outside the mixing area. The rate at which the frit is fed into the furnace is regulated by the rate glass is drawn off from the other end of the `tank’ some 27 metres away.
The furnace itself is constructed of special refractory bricks made from silica or zirconia compounds which can withstand the high temperatures and the corrosiveness of molten glass. They must also heat and expand at the same rate to avoid stress breaks. At the tanks mouth the furnace looks like a huge oven. Viewed across its six metre width through small observation ports it is rather like a swimming pool covered with a low shallow-domed roof. Flames fired by water-cooled natural gas burners blast across the molten liquid’s white hot surface from low arches in the furnace’s walls.
The melting process takes place in three stages. The temperature at the filling end, 1400°C, rises to 1550°C as the fit and cullet move into the tank for initial melting. The glass is then kept above 1400°C to refine it. Bubbles of carbon dioxide gas rise to the surface leaving clear molten glass. Pilkington use about 1300 cubic metres of gas an hour to maintain these temperatures, sufficient energy in just 60 minutes to drive a car from here to England. In the waist of the tank the molten liquid is stirred and a water-cooled pipe extends about 18cm into the liquid glass. It acts as a barrier, trapping any surface defects to prevent them going through to the finished area.
Finally the glass has to be cooled to about 1000°C before it can be worked. “We’ve spent a fortune putting temperature into the glass,” says Brian Armstrong. “Now we’ve got to cool it by blowing cold air into it to get it to the right viscosity to draw it.”
The furnace holds 550 tonnes of molten glass 1175 cm deep. Once it has cooled sufficiently it flows into a small extension to the tank called a drawing canal. The drawing process is started by dipping an iron grille or `bait’, about three-quarters the width of the finished sheet, into the glass. The glass sticks to the bait as it is slowly raised at about 40 metres an hour. Once pulled out to its correct width the edges are gripped between electrically driven, asbestos-covered rollers. The glass solidifies quickly and continues to cool as it is pulled up the tower by rotating asbestos-covered rollers.
The drawing area has five glass operators a shift. They work against the background sound of offcuts and scrap from the break-off area above them rattling and tinkling down the metal chutes outside. The company’s biggest maintenance cost is wear on the two drawing towers’ rollers, caused by high temperatures and glass breakage on them. The edges of the sheet are controlled by nothing more than heat and air. Maintaining the sheet at the correct width (just over 2.5m) and thickness requires discretion and experience. The thick-ness of the glass is determined by the speed of the draw. The faster it is drawn the thinner the sheet of glass. The thinnest glass, 2mm thick, is lifted at approximately 170 metres an hour.
Neil Flintoff, adjusting the pads on the cooler to ensure that the glass is within marketing tolerance of plus or minus 0.2mm, says, “This is not a process that can be exactly calibrated by machinery.” Brian Armstrong agrees: “It is very easy when you are sitting there for eight hours looking through that little observation window to let your mind and concentration go. But if the edges get away from you, you’re making a product that is not saleable.”
When the sheet glass reaches the top floor of the factory it is 10 metres above the molten glass surface, a mere 280°C now and cool enough to be handled. As the continuous sheet rolls up through slits in the floor it is grasped and held by two large rubber suckers. Top floor leading hand, Roddy Alison constantly checks the sheet’s thickness with a substance gauge, his arms covered by protective sleeves made of wire mesh covered with cloth. He is in regular communication with the glass operators on the floor below.
On the top floor the emerging sheets of glass are scored and snapped off by a break-off machine, BOM. The staff often add a ‘b’ to the end because, they say, the glass does break like a bomb. It’s a sound that makes those unused to it flinch. The rubber suction pads lift the cut sheet, which shows an alarming amount of movement and flexibility, away from the next sheet already rolling up. It is automatically placed on more rollers, this time moving horizon-tally. There the edges that had been held by the towers’ rollers, looking a little like selvedges on a length of material, are snapped off and discarded before the still cooling glass is checked and then stacked carefully on frames to be taken down by lift to the warehouse. Each sheet weighs about 22kg.
The factory employs 214 people at present, 68 being directly involved in one of the production shift rosters. It produces 850 tonnes of glass a week of which 30 per cent is lost. The factory’s weekly output is 76,000 square metres, enough to cover 12 rugby fields. The loss is recycled as cutlet. Almost all the glass is sold for general glazing in houses or factories. The remaining five per cent is selected quality glass which is used to make mirrors or toughened for use in domestic appliances like ovens. The company exports about 40 per cent of its production, mainly to Australia, Japan and the Pacific Islands.
Glass for art’s sake
In the 1960s Harvey Littleton, whose father ran the American company Corning Glass, the biggest glass manufacturer in the world, decided that the same thing could be done for glass that the Englishman Bernard Leach had done for pottery decades before: the rediscovery and re-development of glass as a craft. Littleton tapped into the technology of a modern glass factory and took those developments into the universities and design and art schools. The result was an upsurge in interest in what has become known as studio glass.
Sunbeam Glass in the Auckland suburb of Grey Lynn, was one of the first exponents of this new movement in New Zealand. The studio began in 1976, originally to make stained glass. It was set up by John Croucher, James Walker and Eric Eimerson. Croucher finally left this year for a break, after 13 years working with glass. Walker sold his share of the business to Garry Nash and Ann Robinson in 1980 when Sunbeam moved on to hot glass. Nash and Robinson still work there. Walker credits Croucher, Nash and Robinson with promoting glass blowing in New Zealand. But studio glass is not just about glass blowing. There is casting, glass sculpture or moulding and flat glass work.
Garry Nash is considered one of the country’s leading glass makers. He has been working in glass since 1979. Part of what attracted him was the technology. “Hot glass is so technology based, so dependent on machinery. It’s not for the faint hearted… as you realise when you look at the gas bills ($2500 every 30 days plus about $500 for electricity) or the costs you have before you can start. It’s not like pottery. You can’t just go out and buy a glass furnace like you can a kiln. You either make it yourself, as we did, or there are a few people in the world who are experts and very expensive. On top of that, glass blowing is very difficult to learn. All through history it has been known to be. People lose heart trying to learn.”
The Phoenicians appear to have been the first to blow glass, in the first century BC. The essentials of the craft have not changed much since. Garry says the only thing new in 2000 years is wet newspaper, used in shaping the hot glass.
The studio glass-maker looks for different attributes in the glass he or she uses from that made in industrial plants. The mixture must be purer, more ‘plastic’ and therefore easier to work, and the finished pieces should have a higher lustre. Sunbeam have their own recipe. They make about 12 tonnes of glass a year and there is a lot of waste.
“We go to a lot of trouble to get pure glass,” Garry explains. “We can’t use coloured glass again. Parengarenga sand has 10 times more iron than we consider acceptable for reasonable quality glass. At the moment we are using sand from Mt Somers, near Ashburton, but there’s only about a year’s supply left. When that runs out we’ll have to import. New Zealand is too new and volcanic to have good quality high silica sand.” When there were three of them working at the studio earlier in 1988 they used about a tonne of sand every six to eight weeks at a cost of almost $450 a tonne.
When Garry and Ann mix a new batch they use a quarter of a tonne of ingredients. The basis is soda-lime glass, as used in bottles, jars and windows, and comprises silica sand, sodium carbonate as flux to lower the melting point and stop the sand setting so quickly, and limestone (whiting or chalk) which counteracts the water solubility of glass made with just soda and sand and makes it durable. That glass is very stiff to work, very dull and flat with no shine to its surface.
Sunbeam add to it sodium nitrate and antimony to get rid of tiny bubbles called ‘seeds’ (they used to use arsenic but with age it yellows glass); zinc oxide, lead and barium carbonate to give surface shine; borax and alumina to stop the finished pieces cracking in dishwashers and scratching; and potash, another flux, which makes the molten glass easier to work. Selenium and cobalt oxide are used as decolourising agents to counteract the slight green tinge left. This is known as optical glass.
There is a third kind of glass, crystal glass. The soda ash is replaced by potash as a flux and the basic stabiliser becomes lead instead of calcium. The lead makes this glass bend light rays more than ordinary glass does and is the reason why crystal is so often sold in a highly cut form.
Sunbeam do not make crystal glass very often. Cost is a major factor—three per cent lead in their batch doubles its price.
The ingredients are shovelled into a ‘day tank’, a furnace where glass is melted, cooled down to work with and then wound up again and the process repeated. It is the size of a fridge, Garry remarks quite accurately, compared to the massive industrial furnaces as big as buildings. The first load fills the tank, which has been heated to 1380°C (for every 200°C the temperature is raised, the gas consumption is doubled). The frit (mixture) fizzes up into a foam before melting down to a third of its original volume. Then a second batch goes in and maybe a third. It is cooked all night and then let stand for a day at working temperature, 1200°C. The next day it is ready to be worked.
The blowing iron, a steel tube four to five feet long, is dipped in the surface of the molten glass in the centre of the tank, deftly twisted and withdrawn. “It takes a long time to train someone to overcome the panic response to putting their hand in the furnace to gather,” Garry says. “When we’re teaching people about blowing and want them to get some idea of what the molten glass feels like when you gather, we tell them to put a jar of clear honey in the fridge overnight and then try lifting and gathering it with a chopstick.”
The glob of gathered glass is golden white as Garry removes it from the furnace. He moves immediately to the ‘marver’, once a marble slab but now a polished slab of very hard steel. The glass is rolled into a wedged cylinder. Then he does his first and sometimes only blow. He blows down and places his thumb over the mouth piece. The heat of the iron and the glass makes the air expand, forcing a small bubble into the solid glass. “What most people don’t realise is that blowing is only a tiny phase. Most of it is shaping.”
Both Garry and Ann refer to blowing as a “strange dance”. “Movement is very important,” Garry stresses, “because the glass is moving and you have got to be totally in tune with it.” For as long as they are working the glass they must keep it rotating, working against the force of gravity. If they leave it in one position it will start to lose its shape. They also move constantly between the ‘chair’, a bench with long arms along which they roll the rod while shaping it with moulds made of wood or pads of wet newspaper, the ‘glory-hole’ furnace where the glass is re-heated while working, or standing to swing the rod and open out the mouth of a vessel such as a bowl by centrifugal force. Larger pieces will have a second and sometimes a third gathering of glass from the furnace in the early stages.
Glass blowing is physically strenuous and requires a lot of stamina. In making a flat bowl almost a metre in diameter Garry had slumped a thin round of dense coloured glass over the first gather, moulded it on and then taken two gathers over that. There may be two kilos of glass on the end of the rod, and because he cannot get his hands beyond a certain point up the iron, the weight he is manipulating is considerable.
“Glass is a poor conductor of heat,” explains Garry. “It is this property that allows us to blow it. But because of that, when the surface freezes up the centre is still contracting. So it has to be kept hot while working and then cooled very slowly to reduce and control the stress. We use an oven similar to a pottery kiln. The work goes in as it is finished and is cooled overnight, ready to be brought out in the morning.”
Garry also uses cold working techniques. He has built much of his own equipment in a small room up-stairs. He uses air-powered dentist drills and diamond lathes for cutting and engraving, and sandblasting in conjunction with acid to frost the glass.
His brilliantly coloured cylinders are made by blowing a big bubble and then “popping the end like a balloon”. The contrasting plate decorating the outside is slump worked in the kiln and engraved. “You have to go to ridiculous lengths these days to try and find something that hasn’t been done by a Roman a couple of thousand years ago,” he says with a grin. Nevertheless it is his sculptural work which has made his reputation overseas.
Although Ann Robinson also blows glass, she is best known for her work with cast glass. Not many people work in this field and very few make pieces as large as her 17 kg bowls and vases.
Ann likes to think of people using the glass she makes “as functional objects”. She was delighted when one of her huge blue cast bowls (which sell for around $2000) on display at the New Zealand pavilion at Expo was bought by a woman whose first thought was to fill it with strawberries for a dinner party. But she also likes to think of her work “enhancing people’s lives, wonderful things they can feast their eyes on.”
“Blowing glass is a young man’s game really,” she says. “Very soon I’ll only be doing casting. I love blowing. When everything’s going right there’s nothing like it. But it is like being an athlete. You need that concentration to be in peak performance every day.”
Because the technique of casting is still developing there is a lot that can go wrong. “Some shapes are prone to problems. With the tall squared vase the corners keep cracking open. You tend to have a high loss rate with each new design until you find the weakness in the cast or design. So there can be a lot of loss in casting and some real disasters in the kiln.”
Ann casts in a refractory mould, a mixture of plaster, silica and brick dust. The plaster has a tendency to break up in the heat but the other materials hold it together.
“It is lost wax casting,” says Ann, “the bronze casting method where the original shape is made out of wax. The mould is put around the wax and then the wax melted out, leaving a hole. The mould is then heated up and the glass introduced. With bronze you can have a very strong mould because you can smash it off at the end. But with glass you can’t or you would break the piece as well. It must be strong enough to withstand firing but soft enough to remove without damage.”
The mould is filled with finely crushed glass. Because of this it doesn’t need the high temperatures raw materials do to melt and fuse it. The moulds are heated to about 850° to 900°C. Ann looks for different glass properties in casting. Blowing needs a plastic glass with good thermal shock properties to take re-entry into the kiln while it is being worked. Casting requires a glass with low expansion and contraction. She colours her clear cullet with ground up dense colour rods imported from Germany, with straight oxides like cobalt and copper or with colours they have melted at Sunbeam. However, there is always a danger of incompatibility when different glasses are mixed, especially during cooling.
Cooling the huge pieces of cast glass requires great care. If it cools too slowly through the devitrification range, between about 800°C and 650°C, then the glass crystallises. Ann crash cools it through this temperature range by opening the kiln door and bringing the temperature down to about 520°C, the annealing point where the piece will be hard but hot. It usually takes 15 to 20 minutes. Then the kiln is closed again and the cooling process becomes very slow, dropping the temperature a set number of degrees an hour. The thicker the piece the slower the process. Her largest pieces take five days.
When the mould is finally removed the glass surface is worked with an abrasive, usually silicon car-bide paper or stones, which grind down the surface and bring it up to a semi polish.
James Walker is an American who has lived in New Zealand for 15 years. He has worked for a year in a glass studio near Wiesbaden in Germany and is one of the leading flat glass artists in this country. His work is the modern version of the centuries old craft of stained glass. Stained glass has been used in New Zealand buildings since the latter part of the 1800s. The first windows were imported from Europe already made, but it was not long before they were being designed and constructed here. One of the early firms, Miller Studios in Dunedin, only recently closed its doors. Today the field has been left almost exclusively to people like James Walker.
The work is limited and this year he has concentrated on completing a master’s degree in fine arts. He says
You have to go to ridiculous lengths these days to try and find something that hasn’t been done by a Roman a couple of thousand years ago.
This has given him the opportunity to have a more social environment to discuss things. “The stained glass world becomes very narrow,” he says. “You can build yourself a glass prison. There is not a lot of work about and when a job comes up then we all arrive at it, hungry for it.”
Walker has done a number of windows for domestic houses and in the last few years he has had two major commissions. The first was for the new Hutt Valley Energy Board building. His huge 29 foot by 28 foot wall of architectural glass was installed in 1986. The second was installed the same year, another wall of glass above the huge doors of the ceremonial entrance to the Ministry of Foreign Affairs in Tonga.
He has made his own glass in the past but sees little point. As he says, a painter doesn’t make his brushes. “One glass firm in Germany has a catalogue with 300 colours and they’ll make any colour you want. It is a full time profession to blow a good cylinder. Their parents did it and their grandparents before them. It would be like re-inventing the wheel. Most of my work concentrates on my design.”
Once the design for a piece of flat glass work is finalised the glass is scored with a glass cutter, following the line of paper patterns. The glass is carefully snapped along the scratched line. The pieces are laid out and put together rather like a jigsaw puzzle. They are joined by narrow lead extrusions, hence the term ‘lead-lighting.’
The pieces of glass are then assembled in a wooden frame, with more lengths of lead used between each section.
The lead joints are then soldered. “At this point,” says James, “the window is not waterproof and the lead is not a tight fit. A special cement is slopped over both sides of the window and this is then covered with whiting, to help it dry. The excess can then be picked off and the window scrubbed clean. It is,” he adds, “one of the most simple of crafts. That was one of the things which appealed to me.”