Controlling Anisotropy in Heat Treated Glass



Photo of Francis Serruys

Francis Serruys

Director Tech. Sales Support & Business Dev.

Saint-Gobain Glass Corp

Photo of Dr. Romain Decourcelle

Dr. Romain Decourcelle

Saint-Gobain Glass Corp



Anisotropy is also known as Brewster marks, quench marks, strain pattern, leopard spots, Iridescence, etc. Although anisotropy is inevitable when heat treating glass, a major breakthrough took place in the industry by developing an on-line anisotropy visualization equipment. The result is an on-line equipment capable of visualizing and quantifying the level of anisotropy of each single heat treated glass coming out of the tempering equipment. The calculation of the level of anisotropy is based on the RGB photo-elasticity. The optical retardation is determined for each pixel of the source image of the heat treated glass which is obtained with the on-line visualizing equipment. This is done by comparing the RGB value of each pixel with a calibrated scale for the given on-line system. Once the optical retardation is determined at any point of the glass, statistical evaluation can be performed to calculate different metrics such as the mean value. The mean value of the optical retardation is then used to propose a classification. This allows a scientific evaluation and classification of the appearance of anisotropy on single heat treated glass. A major advantage is the consistency of the produced glass against an approved mock-up glass. In a second step it will allow the architect to specify heat treated glass in an unambiguous way. Consequently a clear specification will then avoid confusion and misinterpretation by the contractors.

More measurement data shall be collected confirming the proposed classification of anisotropy. Ultimately the measurement method as well as the classification shall be standardized and become the reference.

Only monolithic heat treated glass was examined till now. The superposition of different glass types will influence the optical retardation of the final product, which will be installed. It shall be examined whether this superposition is important or not on the appearance of the final product. Finally the additional cost of fabrication shall be determined as controlling the level of anisotropy during fabrication influences the overall equipment efficiency.


Anisotropy is also called ‘iridescence’ or ‘leopard spots’ or ‘toughening marks’ or ‘quench marks’ or ‘Brewster Marks’ or ‘strain marks’ (Fig. 1).

Figure 1: Anisotropy visible on a façade.

The European

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Glass Processing and Anisotropy

Anisotropy is the inevitable consequence of the heat treatment process in order to obtain heat strengthened glass (HS) or fully tempered (FT) glass also called thermally toughened safety glass. This

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Measures to Control Anisotropy

The optimisation of the heat treatment process can be evaluated by checking the level of anisotropy after the heat strengthening or tempering process. Ideally this is done for each glass

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Acquisition of On-Line Anisotropy Pictures

Many parameters are influencing the visualisation of anisotropy. The most important parameters are the type of light – monochromatic light or white light – and the kind of polarising filter

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The calculation of the level of anisotropy is based on the RGB photo-elasticity (M. Illguth et. al., 2015). The optical retardation is determined for each pixel of the source image

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From data collected with the first on-line equipment capable of measuring optical retardation of heat treated glass, the following classification is proposed (Table 1). The classification is based on the

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Financial Impact

The sharp point of a triangle will always show critical anisotropy. Moreover the shape of the triangle is influencing the air-flow in the furnace and quench and affects the anisotropy

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Conclusion and Future Work

As anisotropy free heat treated glass doesn’t exist and as the environment is influencing the appearance of anisotropy it is recommended to evaluate the glass by means of a mock-up

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Rights and Permissions

EN 12150 Glass in building – Thermally toughened soda lime silicate safety glass, 2015.

EN 1863 Glass in building - Heat strengthened soda lime silicate glass, 2011.

EN 14179 Glass in building - Heat soaked thermally toughened soda lime silicate safety glass, 2016.

ASTM C1048 – 12 Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass, 2012.

M. Illguth, C. Schuler, Ö. Bucak, The effect of optical anisotropies on building glass facades and its measurement methods, Frontiers of Architectural Research (2015) 4, 119 – 126.

H.W. McKenzie & R.J. Hand, Basic optical stress measurement in glass, Society of Glass Technology, 2014.