Anisotropy as a defect of British architectural float glass heat treatment
The cause of anisotropy and the conditions that may affect it have been reviewed through the literature and the current state of existing production and measurement processes aimed at minimizing its visibility. This shows that the visibility of this phenomenon depends on polarized light, viewing angle, and the stress in the glass plate, the latter being a function of the temperature distribution during the heat treatment.
A few professional glass suppliers seem to have changed their tempering equipment and processes to reduce this phenomenon. State-of-the-art scanners may be able to measure and analyze anisotropy patterns, and enable the special design and operation of tempering furnaces to reduce the visibility of anisotropy phenomena. This is an important step for the glass industry, which has always believed that this inevitable problem cannot be alleviated.
The continuous development of measurement equipment is also critical to objectively defining acceptance and rejection parameters. These may be included in future revisions of regulatory standards and guidelines, where anisotropy is not currently considered a defect, but a visible impact. This is in contrast to many specifications that require glass anisotropy to be reduced or no anisotropy
The results of a detailed survey of 35 key stakeholders (architects, glass suppliers, professional curtain wall contractors, and curtain wall consultants) highlight the relationship between supply chains that accept this phenomenon and designers who desire to reduce glass and their customers. The divergence is anisotropic.
It also emphasizes the need for a detailed explanation of the anisotropy and the deficiencies of the current standards, which lack objective acceptance parameters and specifications, thus attracting protracted qualification reviews and potential controversy. Anisotropy is an important issue for stakeholders who want to receive up-to-date information about the causes of this phenomenon, the state of the industry, and what measures can be taken to reduce its visibility. This research can be used to support such actions and provide information for further research.
Anisotropy is a term used in the exterior wall industry to describe the performance of patterns and colored areas in heat-treated glass under specific light and viewing conditions. This phenomenon is caused by the stress embedded in the glass plate during the heat treatment. The stress level and distribution that make the glass birefringent or birefringent are a function of (non-uniform) temperature distribution and heat transfer. These distributions and heat transfer are affected by, for example, but not limited to, the glass plate on the support roller, the glass plate The swing, the arrangement and operation of the heating tube and the quenching nozzle (note that the air floatation technology tempering was not reviewed in this study).
Early research by Redner and Bhat  emphasized the uneven stress distribution caused by the tempering process. This leads to complex thermodynamic processes and complex glass area layouts that change the behavior of (polarized) light in relatively different ways, resulting in specific (anisotropic) patterns.
Polarized light, that is, light that only oscillates in one plane, is a naturally occurring and inevitable phenomenon caused by natural light reflection and scattering. Illguth et al. provide detailed explanations of these optical phenomena.  also proposed a method to measure anisotropy, Henriksen and Leosson  also discussed the reasons and provided some examples of specific anisotropy patterns, and proposed a method to quantify the phenomenon.
Technologies and equipment that promise to measure and reduce visible iridescence have been on the market in the past few years. On the contrary, a more uniform temperature distribution during the entire heat treatment process will result in a more uniform stress distribution, thereby reducing the interference anisotropy mode.
Figures 1 and 2 show an example of a leading glass supplier's internal modification of a tempering furnace to minimize the visibility of anisotropy. The change in roller design creates a more uniform heat distribution and eliminates the central longitudinal band (shown by the red arrow in Figure 1), resulting in a more uniform and, more importantly, less disturbing anisotropic mode (Figure 2) .
Current glass industry standards and guidelines recognize that anisotropy is an unavoidable characteristic of heat-treated glass when exposed to certain light conditions. For example, see CWCT Technical Note 35 (Evaluation of Glass Appearance ) and British Standard Glass Process (BS EN 1863). :2011 ).
The exterior wall specifications have always been consistent with these standards, but in recent years, due to frequent requests for glass with reduced anisotropy or non-anisotropic glass, and by effectively redefining glass defects that may be rejected, the specifications have become more and more frequent. The more stringent. On the contrary, due to the limitations of glass processing, the exterior wall industry is striving to meet these needs, which limits the actual production capacity of the industry.
In addition, factory and on-site glass acceptance and rejection due to anisotropy are basically based on subjective inspection standards and are supported by extensive sampling because it is not clear whether and how to measure or control anisotropy. This phenomenon is usually the subject of divergence, and may eventually lead to disputes between the cladding supply chain that does not think it is a defect and the designer/specifier who thinks it is a defect and wants to avoid it and their customers.
This work aims to determine the extent to which architects, glass suppliers, professional curtain wall contractors and curtain wall consultants regard the anisotropy of heat-treated glass as a defect, and how this affects the entire British curtain wall industry. It presents the main results of a survey of stakeholders and attempts to determine the level of clarification required, the current state of the industry, while reviewing future challenges and possible directions.
The background of this research is to seek a clearer and simpler definition of anisotropy as a phenomenon and its causes. To achieve this goal, we have conducted a comprehensive review of glass as a building product and related literature, paying special attention to the process that leads to anisotropy. This has resulted in an assessment of the current state of glass manufacturing technology and, most importantly, whether and how to measure and control the visual disturbance anisotropy in architectural glass. The resulting industry background is compared with existing industry standards, guidelines and norms.
A preliminary pilot survey highlighted the tension between the current specifications for non-anisotropic glass and the challenges faced by the exterior wall industry that relies on the glass industry to meet such needs. The results of this preliminary pilot survey are used to provide information for a more detailed survey, which aims to assess the knowledge and perceptions of key exterior wall industry stakeholders related to glass anisotropy. The results determined the severity of the problem, highlighted any related shortcomings, and showed how different stakeholders are currently dealing with anisotropy. This is used to determine possible and sustainable ways to move forward, and it is possible to improve standards and guidelines related to the anisotropy of architectural glass.
The survey was conducted on 37 industry stakeholders in March/April 2014, including architects (10), exterior wall consultants (9), glass suppliers (8) and professional exterior wall contractors (10). name). 35 questionnaires were returned, and the overall response rate was 95%, indicating that the industry is quite interested in this phenomenon. With the exception of one interviewee, everyone else expressed interest in obtaining more information about the phenomenon and its impact on the external wall and glass industry. It’s worth noting that the six interviewees wanted to remain anonymous: four of them were suppliers, and they might be reluctant to share their views publicly.
At least 60% of participants believe that this phenomenon is critical or important in the tendering, production and handover stages: after the project is completed, this does not seem to be a problem (43%) (Figure 3).
Although most survey respondents (43%) believe that the problem of anisotropy in glass can be solved, there is considerable uncertainty, because more than one-third of people are not sure whether this is the case, and 20% People think it cannot be solved (Figure 4). These responses confirm the chaos and division of the industry because it is related to this phenomenon.
A major glass supplier is currently selling glass with reduced anisotropy, and at least another R&D effort as previously shown has achieved satisfactory results. Private communication with the supply chain confirms that operations, expertise and maintenance are essential for obtaining the best results. Contrary to the historical position of the glass industry, it seems likely to affect the visual impact of this phenomenon.
For 69% of the respondents, current industry standards and guidelines are not sufficient to regulate anisotropy, and only 4 respondents think they are. Anisotropy is considered to be the inevitable influence of heat treatment processes, and the current British standards regulate such processes (ie BS EN 1863:2011 , BS EN 12150:2000 , BS EN 14179:2005  ) And the glass industry guide.
The reviewed documents seem to have consistent interpretations of this phenomenon, which are also called leopard spots, iridescent, quench marks, or strain patterns. Of the four documents reviewed, only one is a British Standard (BS EN 1863:2011), which clearly states that anisotropy is not a defect. However, this important statement is expected to appear in the next revision of the British tempered glass standard (BS EN 12150) and may help improve the clarity of the standard.
This seems to be in contrast to the current desire of customers and their designers to purchase glass with no obvious anisotropy. Further revisions to glass standards may also need to consider the progress made by the glass industry in evaluating and reducing this phenomenon. It is best to list the measurement process and the anisotropic acceptance and rejection criteria and parameters.
In addition, it should be noted that nowadays glass is hardly used as a whole product. Heat-treated glass can be coated, welded, laminated and used in combination to form complex products, and these products can be used to form complex double-layer and triple-layer insulating glass units. These numerous and different configurations may affect the visibility of anisotropy, so they should be mentioned in the relevant standards, not currently available, and comprehensive guidelines. A detailed assessment of this impact may require specialized research, but this is outside the scope of this research.
Although glass industry standards and guidelines do not define defects as defects, 26% of respondents regarded anisotropy as defects. Perhaps not surprisingly, further analysis of the data shown in Figure 5 shows that architects and exterior wall consultants accounted for 78% of those who claimed that the phenomenon was a defect. For these two groups, architectural aesthetics is the most important, so the requirements for quality are higher. On the contrary, the location of the remaining parts is consistent with the current glass regulatory standards.
The majority of respondents (51%) confirmed that their customers are not prepared to pay more to avoid problems or take risks related to anisotropy. Depending on the type and specifications of the contract, in this case, the risk can be easily transferred to the supply chain "for free". On the other hand, slightly more than a quarter of participants (26%) indicated that customers may be prepared to pay more to reduce the risks associated with anisotropy, while 23% of participants did not know.
There is a current trend to avoid the use of heat-treated glass to avoid anisotropy problems, and 40% of respondents confirmed this. Since this product is suitable for multiple applications (for example, increased strength in the case of fully tempered glass, safety fracture, sintering/enamel panel processing, holes and notches), it is not always possible to completely avoid heat treatment of glass, so it is not a solution A sustainable approach to anisotropy issues.
The specific investments and research of some industry participants seem to confirm that the visibility of this phenomenon is still inevitable and can be changed and minimized to produce glass with less disturbing anisotropy. However, the scale of related costs is still unclear, and special market research may be required. The design of the tempering furnace and the temperature and stress distribution are critical to solving the visibility of the pattern, as is its operation and maintenance.
The development of anisotropy measurement equipment is crucial to analyze the stress and its distribution, and then optimize the tempering furnace design and heat treatment temperature distribution to reduce the visibility of anisotropy. Measuring equipment is also the basis for defining objective acceptance and rejection parameters. These can be used not only for benchmarking, sampling, and quality control, but also for redefining regulatory standards if the entire industry agrees.
The current standard is being revised to clarify that anisotropy is not a defect. On the other hand, the glass industry, or some of its major players, has made good progress in quantifying and minimizing this phenomenon and its impact, so it is possible to move towards satisfying the wishes of customers and designers. This may cause a deadlock, which can be temporarily resolved through guidelines and technical instructions, but ultimately requires future revisions to the standard. Unfortunately, this can be a lengthy process because of the natural delay in the consultation period for revised documents before approval and final publication.
The survey results confirmed that the exterior wall industry is divided into supply chains that accept anisotropy and designers/regulators seeking to reduce anisotropic glass. This matter is very important to stakeholders. Although the progress and products of a limited number of suppliers seem to be able to meet their glass aspirations, customers seem to be reluctant to pay additional costs. The survey response confirmed that there is a tendency not to specify heat-treated glass to avoid anisotropy problems. If anisotropy is considered a defect, the regulatory standards are currently inadequate and may need to be updated to reflect current industry developments and include objective acceptance and rejection criteria used in the specification.
If the anisotropy problem is not resolved to the satisfaction of all parties, the current situation will only get worse and more chaotic, because the demand for almost defect-free glass becomes more and more common. Conversely, the tempering furnace supply chain industry may turn problems into innovation opportunities
 Pasetto, S., 2014 Anisotropy as a defect of heat-treated glass in British architectural float glass, Master's thesis of University of Facade Engineering, Bath, UK  Redner, AS and Bhat, GK, 1999 Advanced method of measuring surface pressure Use of readings based on digital image analysis in the proceedings of glass processing days, June 13-16, 1999, Tampere (Finland): Glass Processing Days, pages 671-673  Illguth, M. et al. The influence of optical anisotropy on architectural glass curtain wall and its measurement method. Frontier of Architecture Research (2015), http://dx.doi.org/10.1016/j.foar.2015.01.004  Henriksen, T. and Leosson, K., 2009 Anisotropy and Optical Distortion of Architectural Glass, Can I Control Glass Performance Day Conference Proceedings, June 12-15, 2009, Tampere (Finland): Glass Performance Day, pages 834-839 [ 5] CWCT Technical Note No. 35, 2003 Appraisal of the appearance of glass. December 2003 British CWCT  BS EN 1863 Part 1:2011 and Part 2:2004 Building Glass-Thermally strengthened soda lime silicate glass BSI  BS EN 12150 Part 1:2000 and Part 2:2004 and prEN 12150- 1:2012 Architectural Glass-Thermally Tempered Soda Lime Silicate Safety Glass BSI  BS EN 14179 (Part 1 and Part 2): 2005 Architectural Glass-Hot-dip Thermally Tempered Soda Lime Silicate Safety Glass BSI
The author would like to thank all survey participants, especially Dr. Steve Lo from the University of Bath.
Special thanks to Arcon and Glaston for invited seminars, private exchanges and support.