Heat-Treated Glass

The Designer’s Guide to Navigating Industry Trends and Minimizing the Appearance of Optical Distortions in Exterior Heat-Treated Glass



Photo of Reece Poth

Reece Poth

Project Consultant

Simpson Gumpertz & Heger


Photo of Conner Claus

Conner Claus

Simpson Gumpertz & Heger




What was once an accepted reality in heat-treated exterior glass, optical disturbances in glass are under increasing scrutiny by developers, designers, and manufacturers. Although dimensional distortions that affect the appearance of heat-treated glass, such as bow/warp, are clearly captured, quantified, and controlled in glass manufacturing standards (e.g., ASTM C1048, EN 12150 etc.), optical distortions due to strain patterns, collectively known as “anisotropy,” are acknowledged but not formally controlled. Larger glass module sizes, now the prevailing style in facade architecture, have required more glass to be heat-treated, forcing the industry to understand and address optical distortions.

The industry is faced with many questions, such as:

  • What are the physics behind optical distortions in heat-treated glazing?
  • How can understanding the physics behind optical distortions in exterior glazing help influence glazing design?
  • How is the glass manufacturing industry adapting to provide glass that is aesthetically acceptable?
  • How can designers reasonably specify aesthetic acceptability criteria of exterior glazing if some visual distortions are qualitative?

To address the above questions, this paper provides a guide for the material design properties of heat-treated glass, including its manufacturing process, manufacturing standards, and recommended specification practices that may be used to lower the likelihood and appearance of optical distortions.


Imagine you are a building design professional. You are walking on a crowded sidewalk downtown and somehow you are early for your next project appointment. Instead of stopping off at

Members Only


No, this is not a physics paper. This paper is not going to “shatter” your previous misconceptions on glass materiality and physical limitations. But to navigate the perils of funny-looking

Members Only

Heat Treatment of soda-lime glass

As we mentioned at the beginning of this paper, we address optical distortions due to tempering glass. In the previous paragraph we have established why HS or FT glass is

Members Only


ASTM C1048-18 Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass [ASTM C1048-18] describes anisotropy as:

7.2 Strain Pattern—A strain pattern, also known as iridescence, is inherent in all heat-strengthened and

Members Only

Heat Treatment and Surface Distortions

Heat-treated glass also produces changes in surface geometry, creating optical distortions. The phenomenon is most noticeable when we observe reflections (e.g., from an adjacent building) on a glass lite. Reflections

Members Only

Industry and Manufacturing Trends

Manufacturers have adapted to client’s and consumer’s increased scrutiny to blemishes and optical distortions in glass. The expectation of greater quality of building materials is creating a demand for product

Members Only

Glass Design

Specifying glass is as much of a science as it is an art. As a design professional, you may have various consultants suggesting to optimize your glass for structural performance

Members Only


Surface geometry and anisotropy distortions are an inherent part of the heat-treated glass established in ASTM C1048. Although the industry has set limits to surface geometry distortions, it acknowledges the

Members Only

Rights and Permissions


ASTM International Committee. "Standard Specification for Flat Glass." ASTM International C1036- 16

ASTM International Committee. "Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass." ASTM International C1048- 18

ASTM International Committee. "Standard Specification for Laminated Architectural Flat Glass." ASTM International C1172- 19

ASTM International Committee. "Standard Test Method for Measurement of Roll Wave Optical Distortion in Heat-Treated Flat Glass." ASTM International C1651- 11 (Reapproved 2018)

ASTM International Committee. "Standard Practice for Determining Load Resistance of Glass in Buildings." ASTM International C1300- 16

Dr. Stephen Lo and Saverio Pasetto. " Anisotropy as a defect in U.K. architectural float heat-treated glass." University of Bath Department of Architecture and Civil Engineering MSc Facade Engineering Procedia Engineering (2014)

F. Serruys, Dr. R. Decourcelle. “Controlling Anisotropy in Heat Treated Glass.” Facade Tectonics 2018 World Conference

GANA Tempering Division – Construction Subcommittee and approved by the Tempering Division – Standards and Engineering Committee and GANA Board of Directors. " Quench Patterns in Heat-Treated Architectural Glass” Glass Informational Bulletin GANA TD 05-0108.

G.N. Srinivasan, G. Shobha. “Statistical Textual Analysis.’ World Academy of Science, Engineering and Technology International Journal of Computer and Information Engineering Vol:2, No:12, 2008

Gregor Saur and Henning Katte. "Inline measurement of Residual stresses in large format objects." Glass Worldwide

H. Aben, C. Guillemet. “Photoelasticity of Glass”. Springer- Verlag Berlin Heidelberg. 1993

Hermann Dehner. "How To Specify Anisotropy in Facade Glass”. GlassCon Global

International Code Council. International Building Code. 2015

L. Hidalgo, M. Elstner. “Anisotropic Effect in Architectural Glass, New measuring and monitoring approaches.” Facade Tectonics 2018 World Conference

LiteSentry. "What is a milliDiopter?" LiteSentury Fact Sheet 2017

M. Illguth, C. Schuler, and O Bucak "The Effect of Optical Anisotropies on Building Glass Facades and Its Measurement Methods." Frontiers of Architectural Research. April 08, 2015.

NSG Group. Installation of Heat Treated Pilkington Clear and Tinted (Non-Coated) Flat Glass. Technical Bulletin ATS-186 (2013)

NSG Group. The Appearance of Quench Marks in Heat Strengthened and Tempered Glass. Technical Bulletin ATS-157 (2013)

Oldcastle Building Envelope. "Heat-Treated Glass." (2011)

Riku Farm. "Towards Better Anisotropy." Glastory. June 10, 2019. https://www.glastory.net/towar... (Accessed July 24, 2019).

Schittich, Christian. Staib, Gerald. Balkow, Dieter, Schuler, Matthias. Sobek, Werner. Glass Construction Manual, 2ndrevised and expanded edition (Birkhäuser Architecture, 2007).