Glass manufacturing improves as raw materials range expands.

The range of raw materials used in glass manufacturing has grown in recent decades. Bill Simpson(*) reviews some of the changes in batch materials for refining and colouring glass.

Involvement in the glass industry during the past 50 years has seen many changes in the use of raw materials.

Dolomite is normally used in flat glass, tubing and lighting glass and for a period it was added to container glass compositions when alkali levels were reduced for economy and higher forming speeds.

Niche market

For the past 13 years Glassworks Services has filled a niche in the market by supplying many of the minor ingredients. When sales were reviewed recently, the company found that it had supplied 50 different materials since its establishment in 1987.

In some cases, a full review of a customer's batch material requirements is carried out and then alternative materials are considered. Different sources are evaluated to give the customer a better economic package with no reduction in quality and service. In fact, in some instances, the company has been asked to look after the sourcing and logistics of all the materials required.

Glassworks Services works closely with companies who supply a wide range of industries and require an outlet into the glass industry. The suppliers often need technical expertise on materials for manufacturing glass, but they cannot justify employing a glass technologist or a specialist for sales into just one of the industries they serve.

Material groups

Materials can be divided into groups such as melting aids, refining, oxidising or reducing, colouring and/or decolourising. In some cases they can perform more than one function.

Typical of such a material is calumite, which is a beneficiated blast furnace slag initially introduced into the UK and Europe in 1968. In addition to being an economic source of alumina, it helps the batch to melt faster and significantly improves refining. The refining action is derived from its sulphide sulphur content, which is also useful when producing amber glass, with the amber chromophore being ferric iron and sulphide sulphur.

Another melting aid is borax, mainly used as the pentahydrate, which has superseded the decahydrate, thus reducing the water added to the glass and the energy required to drive it off. However, the dehydrated form is now being widely used with its lighter batch weight and lower dust level and is less fluffy when heated. It has benefits for the glassmaker even if a premium has to be paid. The bar chart indicates the proportion of water, which has to be driven off when using the hydrated borax (fig. 1).

                      Anhydrous   Penta     Deca

[Na.sub.2]O            30.80      21.20    16.30

[B.sub.2][O.sub.3]     69.20      47.90    36.50

[H.sub.2]O                        30.80    47.30

Lithia grades

Lithia is a melting aid, which has found favour in some types of glass. A lot of work has been done with two grades of spodumine, which have 4.5% and 7.5% Li[O.sub.2] content. It is also available as petalite with 3.4% Li[O.sub.2] and lithium carbonate. The latter has become much more competitive and performs better during the melting process, although in which form it is added makes no difference to the final glass properties.

Prior to the advent of calumite, most sodalime glass contained sodium sulphate for refining. It also contained arsenic trioxide and sodium nitrate for decolourising and to some extent for refining. Those glasses unable to use calumite also moved to a less oxidised batch and added carbon, usually in the form of anthracite, removing the arsenic and nitrate. Some tableware glass retained the nitrate and most added ceria or neodymium oxide to improve the colour

For some time, calcium sulphate (anhydrate) was used but about 10 years ago production was stopped, which luckily coincided with sodium sulphate becoming widely available. In some special glasses the sulphate is introduced as barium sulphate.

Refining actions

Sodium nitrate is still used as an oxidant but its use is rather limited. Potassium nitrate is used in lead glass, normally in conjunction with arsenic trioxide in full lead crystal and with antimony trioxide in 24% lead glass. Both have refining and decolourising actions.

Other oxidants used for efficient decolourising are cerium oxide (95% to 98% Ce[O.sub.2]) and cerium concentrate (usually with 75% Ce[O.sub.2] content). The ceria also combines with iron as a chemical decolourant. It is used in pharmaceutical tubing and TV glass as there is no colouration when the glass is irradiated, unlike when arsenic was used and the glass turned brown. It is also used in combination with titania to give a very stable yellow/amber (depending on the concentration), which is unaffected by the base glass or the redox state of the glass or furnace.

Rare earth products

Other rare earth products used for colouring or decolourising are neodymium oxide and more recently erbium oxide. During all the considerations for producing a non-lead crystal, it was determined that all toxic materials should be eliminated from the batch, hence the discarding of baria as a constituent and selenium as a decolouriser. It was found that erbium oxide gave the best decolourising results and is unaffected by the redox state of the batch and furnace. It is now used in high quality tableware and similar soda-lime glasses with excellent effect, while some glassmakers are using neodymium oxide quite successfully and at lower cost.

In amber glass where additional iron is required it has been found that the black glassy iron silicate is to be preferred to the dusty iron oxide, as it is in a form easily assimilated into the melt. It is about a third less expensive to use than the oxide. Other relatively new materials to be used in amber glass production is where lightweighting of a bottle makes a normal amber to light and there is a need to make a dark amber. In this situation both copper oxide and cobalt oxide have been added depending on the shade of amber required.

Minor ingredients

There are many other minor ingredients used in the glass industry including vanadium pentoxide for an infrared absorbing glass, cadmium compounds for yellow and red glass and sodium silicofluoride as a flux.

For many years selenium was added to a glass as a 300 mesh product, the idea being that a metallic powder needed to be very fine so that it would be readily fluxed into the melt. However, there was a tendency to use zinc selenite (40% Se), which it was claimed had a better retention in the glass. Progress has been made in the past decade whereby glassmakers are using coarse selenium (0.23/1.00mm) because it is less dusty and the retention in the glass is much improved and more economical.

Cobalt oxide is still used in conjunction with selenium for decolourising bulk soda-lime glass and for producing a rich blue glass.

(*) Bill Simpson, managing director Glassworks Services Ltd, Doncaster, UK. Fax +44 (0)1709 770803.