soda lime glass being made

Soda-Lime Glass: Production Process and Method of Coloring

Glass is one of the most useful materials used in manufacturing industries because of its versatility in producing a variety of items for household and industrial use. It is useful for the formation of lenses, glasses, window panes, kaleidoscopes, containers, and much more.

It is arguably humankind’s most important material as its uses are limitless. It owes its versatility to unique properties such as its insulating properties, strength, low melting point as well as reflective, transparent and translucent properties.

There are two primary categories of glass making, namely fiberglass and soda-lime glass. For the most part, fiberglass is a material produced from extremely fine fibers of glass. Its application cuts across a wide range of industries such as household insulation etc. It is also used as a reinforcing agent in automotive body panels and ladders.

At the same time, soda-lime also has a host of applications. In fact, it is the most common form of glass produced and is used in 90% of all glass applications.
Soda-lime glass is composed chiefly of silica (silicon dioxide) which makes up about 70% of its constituents. It also has a fair share of soda at 15%, lime at 9% and little amounts of various other compounds.

molten soda-lime glass being poured
sheet glass being made

To reduce the melting point of the silica, soda is added to act as a flux and thus reduce the temperature. Similarly, lime is added to the silica to act as a stabilizer for the compound.

It has the benefits of being relatively inexpensive, reasonably hard, chemically stable and extremely workable. Its workability makes it easy to be re-softened as many times as possible if needed to finish an article or item.

These qualities and benefits makes it suitable for manufacturing a wide array of glass products including art objects, bottles and light bulbs.

Soda-lime glass is commonly manufactured via a wet batch process where about 60-75% silica, 12-18% soda and 5-12% lime is used.

In most cases, small amounts of other materials are also added to make changes to the color of the product. Materials such as chrome flour, iron pyrite, copper oxide and red iron oxide can be used to add color to the product. For instance, cobalt is used to make blue colored containers.

Asides its use as a stabilizer for the silica, lime is also used as a flux for modifying viscosity and devitrification, as well as increasing the final product’s durability and chemical wear resistance.

Perhaps the most essential material in increasing the durability and chemical wear resistance of the final product is milled limestone. In the absence of milled limestone, soda-lime glass will dissolve in water.

a range of different coloured glass

Production of Soda-Lime Glass

In general, soda-lime glass is of two types – flat glass/ float glass and container glass.

The making of soda-lime glass is in four phases: These phases include preparation of raw material, melting in a furnace, forming and finishing.

Once the soda ash materials, sand and limestones are received, they are ground into powder and stored. Thereafter, the grounded mixture is moved to a special gravity feed system, where they are then thoroughly mixed together by a mixer. To ensure a uniform mixing, cullet may be added to the mixer.

Subsequently, the mixture is conveyed to a storage bin. Here, the mixture is slowly fed into the melting furnace.  A batch plant which refers to the entire arrangement of all the tools and equipment involved in the handling and preparation processed, is often located away from the main furnace.

sheets of window panes
Container Glass Bottle Chrome Flour Iron Pyrite Red Oxide Copper Oxide Anthracite Redox Glass Carbon

In most cases, the commonly used furnace is the continuous regenerative type. The furnace typically used may have end ports or either side that connect brick checkers internally to the melter. Brick checkers are the more economical option as they help to save fuel. This is made possible by using the furnace gas heat to initially heat the combustion air before it is fed into the furnace.

The material is fed to the melting furnace through a feeder. A Melter helps to melt the material as it passes through it, and the molten glass is the led to the refiner. The refiner helps to treat the molten glass in a heat conditioning process. This process is crucial, as the glass cannot be formed without first heat treating it.

Having refined the molten glass, it leaves the furnace through forehearths with the only exception being in the float process. In the float process, the molten glass is delivered to the tin bath instead. The molten glass passes through the forehearths to be formed into the desired shape. Subsequently, the molten glass is shaped by pressing, blowing, pressing and blowing, followed by rolling, floating or drawing into the desired form.

For the most part, the pressing and blowing processes are performed mechanically. This is done using blank molds and gobs (glass cut into sections) by a set of shears. For the drawing process, the molten glass is drawn upwards usually through rollers. The thickness of the glass sheet formed is determined by the speed of the draw and the arrangement of the drawbar. The rolling process also shares a host of similarities with the drawing process.

The only exception is that the glass is drawn horizontally on patterned or plain rollers, and in plate glass requires some more grinding and finally, finishing. For the float process, a molten bath is used. The glass is drawn and formed - over a molten tin bath – into a finely finished surface. This finished surface often doesn’t require additional grinding or finishing.

The produced glass then undergoes finishing (coating or decorating) as well as annealing – the removal of unwanted stress areas in the glass. It is then inspected and readied for delivery to the market. If any of the products are deemed undesirable or damaged, they are transferred back to the batch plant where they can be once more be used as cullet for another glass making process.

red hot glass bottles
bottles on an assembly line

Coloring of Soda-Lime Glass

Soda-glass in its pure form has no color. However, small impurities of iron glass can tint the glass green, and this tint can be especially visible in thicker glass pieces.

To make colored glass, powdered metal oxides, sulfides, or other molten compounds are added to the molten glass mixture. Hence, enhancing its aesthetic appeal.

Examples of coloring ion used to change the color of glass include:

Sulphur: Together with iron and carbon salts, sulphur forms iron polysulfides which is used to make amber glass. The concentration, redox state and nature of the compound used are significant factors that define the color of the glass. Iron pyrite is a good source of Sulphur to produce amber glass

Iron (II) oxide: Iron (II) oxide can be added to glass to produce a bluish-green coloring. This sort of glass is what is commonly used for beer bottles. When chromium is further added to the mix, it produces a more vibrant green color, widely used for wine bottles.

Chromium: It is known to be a potent colonizing agent. It produces dark green or even black when used in higher concentrations. When used in a mixture with the combination of arsenic and tin oxide, it yields emerald green glass. A good example in this application is chrome flour.

Copper oxide: 2 to 3% of copper oxide added to molten glass produces a turquoise color.  It is also used in the production of black glass which involves a heat strike

clear glass bottles
various coloured bottles

Manganese: When added in small amounts, it helps remove the green tint given by iron. When put in higher concentrations, it provides an amethyst with color to the glass instead. Manganese is one of the earliest additives used for glass coloring. Specifically, purple manganese glass was used in early Egyptian history.

Cobalt: When small concentrations of cobalt is added to glass, usually between 0.025 to 0.1%, blue glass is produced. For best results, glass containing potash should be used.

Nickel: Depending on its concentration, addition of nickel can produce blue, violet or in some cases black glass. When lead crystal is added with nickel, a purplish color is acquired. Nickel together with small amounts of cobalt is used for decolorizing cobalt glass.

Cadmium: Together with sulphur, it produces cadmium sulfide, resulting in deep yellow color often used in glazes. That said, cadmium can be very toxic. When used together with sulphur and selenium, shades of bright red and orange are produced.

Titanium: Titanium produces a yellowish-brown glass when added. With that being said, titanium is rarely used for coloring glass. Instead, it is used to intensify and brighten other coloring additives.

Didymium: It is used in addition with glass to produce a green or lilac red color.

Uranium: When added to glass, it yields a green or fluorescent yellow color. For the most part, uranium glass isn't radioactive enough to be dangerous. However, if ground into powder – which can be achieved by polishing sandpaper – and inhaled, it can be carcinogenic. When used in combination with lead glass with high concentration of lead, a deep red color is produced.

Manganese dioxide: This additive which has a black color, is used to remove the green color from glass. In what is usually a slow process, manganese dioxide is converted into sodium permanganate which appears as a dark purple compound.

red iron oxide powder in a pot
Pyrites powder in a pot
Chromite Flour in a pot