Conditioning and Forming


After completion of the refining stage the fairly homogenous, bubble-free glass leaves the tank and enters the forehearths – the main channels that transport molten glass to the forming machines. The main function of forehearths is to condition the glass; that is producing a stable glass with desired glass temperature, evenly distributed both vertically and laterally, to be delivered to the forming process. Conditioning in the forehearth is of critical importance, as deviations from the desired thermal profile can cause undesirable differences in viscosity, and subsequently lead to visible defects in the finished product. Performance of the forehearth is rated by the range of pull rates and gob temperatures where the production can maintain an acceptable degree of homogeneity, the speed of response of the forehearth, and its ability to maintain temperature stability. Its roofblock shape, the number, the position and the size of exhausts, the degree of controllability of the combustion and cooling exhausts, and uniformity in temperature and viscosity distribution are important parameters in designing an efficient forehearth. Forehearths can be gas-fired or electrically heated. In general, electric or new forehearths are more energy efficient than older models (Worrell et al., 2008. pp.11 & 71)


The conditioned glass is delivered from the forehearth to the forming equipment at a constant rate, also known as "pull rate". Depending on the process, the viscous glass stream is either continuously shaped (floatglass, fiberglass), or severed into portions of constant weight and shape (“gobs”) which are delivered to a forming machine (container glass).

In container glass production, the molten glass stream leaves the forehearth though an orifice ring at a constant rate, and is severed into portions of defined weight and shape, known as "gobs" by mechanical means. The gobs are delivered to the forming machine and are given the shape of the final product by automated processes known as pressing, blowing, press- blowing, and blow-blowing.

Flat glass is produced today either by the float glass process, continuous drawing (updraw, downdraw, overflow fusion), or continuous rolling. In float glass process, the conditioned glass flows onto the surface of a molten tin- alloy (“float bath”) after leaving the delivery system. The temperature at the entrance of the float chamber is high enough (typically around 980°C to allow the glass to spread out on the liquid metal bath and form a flat ribbon, and remove irregularities in the surface figure. The ribbon is continuously withdrawn from the float chamber, and cools while floating on the tin-alloy bath to about 590°C; the glass is then rigid enough to be lifted from the float bath without deformation and surface damage by conveyor rollers.

Glass fiber consists mainly of continuous glass fiber (e.g. textiles) and glass wool (used for insulation). Continuous glass fiber is a continuous strand, made up of a large number of individual filaments of glass. It is produced by feeding the molten glass to a series of bushings that contain over 1,600, or more, accurately dimensioned holes, or "forming tips”, through which fine filaments of glass are drawn mechanically downwards at high speed, and are wound. In producing glass wool a thick stream of glass flows by gravity from the bushings into a rapidly rotating alloy steel dish "crown," which has several hundred fine holes around its periphery. The molten glass is ejected through the holes by centrifugal force to form filaments that are further extended into fine fibers by a high velocity blast of hot gas. These filaments are sprayed with a suitable bonding agent and drawn by suction to form a mat of tangled fibers. This then enters an oven where the bonding agent is cured and is then cut to size.

Optical fibers are produced using extremely pure glass and there are many manufacturing processes being used for their production. For extremely accurate dimensions and complicated inner and outer profiles, extrusion is used to form the glass. Extrusion uses low process temperatures and a glass melt with unusually high viscosity compared to traditional forming methods like drawing or blowing (Worrell et al., 2008. pp.11-12).

Conditioning and FormingTechnologies & Measures

Technology or MeasureEnergy Savings PotentialCO2 Emission Reduction Potential Based on LiteratureCostsDevelopment Status
Oxyfuel Fired Forehearths in Fiberglass Production

In two different plant tests, the technology has shown to reduce foreheart energy consumption by 56% and 64%, saving 81 TJ/y of natural gas (Mighton, 2007. p.6).

In the test plants, the system eliminated 4500 tons/yr of CO2 emissions (Mighton, 2007. p.6).

Estimated payback time is 2.2 years (Mighton, 2007. p.6).

Better Control of the Tin Bath Temperature in Float Glass Production


Quantitative information not available. 


Quantitative information not available. 


Quantitative information not available. 

Forehearths Process Control - Infrared Analysis System

Typical energy savings are estimated at 2-3% of total plants energy use through reduction of the reject rate. A plan in Netherlands was able to reduce fuel consumption of the plant by 5% (Worrell et al., 2008. p.71).

The installation of this system provided a plant in the Netherlands with total annual benefits of over $3 million/year, of which $200,000 was due to energy savings (Worrell et al., 2008. p.71).

More Efficient Forehearts

By installing the efficient design of forehearth, a plant in Norway was able to replace 3000 MWh/y natural gas consumption by 350 MWh/y of electricity consumption (~ 1078 MWh/y primary energy), providing an energy saving of 65% – in terms of primary energy (Worrell et al., 2008. p.71).

In the Norwegian plant, installation of efficient forehearths required US $ 120 000 investment and provided annual savings of $ 95 000 – resulting in a payback time of 1.5 years [1987 values] (Worrell et al., 2008. p.71).

Forehearths Process Control - Continuous Gob Monitoring System

By installing this system, a glass container plant in the UK realized primary energy savings of 2.4 TJ/year (Worrell et al., 2008. p.71).

The plant in the UK realized savings of $8,600/year due to reduced energy cost and $8,500/year due to reduced material costs. With a total investment of $26,000, the payback time was 18 months [1993 values] (Worrell et al., 2008. p.71).

Forehearths Process Control - Advanced Adaptive Process Control

Specific energy savings are enabled due to production increases of 3.75 to 20% for common containers  and up to 40% for speciality containers (Worrell et al., 2008. p.70). 

Paybacks for the advanced process controller system are estimated to be 2 to 9 months (Worrell et al., 2008. p.70).


Conditioning and Forming Publications

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Conditioning and Forming Reference Documents

Best Available Techniques (BAT) Reference Document for the Manufacture of Glass

As a reference of the EU Industrial Emissions Directive (2010/75 EU) this new version provides extensive information on Best Available Techniques (BATs) applicable to European Glass Manufacturing Industry for reducing environmental impact. The document is prepared by the  Institute for the Prospective Technological Studies of European Commission's Joint Research Center. 

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