Glass standards and quality for architectural glazing

23 May 2021

Minimalist VIIU sliding doors by Exact Architectural Glazing in Wandsworth home

The specification and quality of glass used in all architectural glazing is critical to ensuring each installation meets the required performance standards. Each pane of glass is inspected by one of our expert team to ensure our stringent quality standards are met. 

Our Q&A aims to answer and explain the technical details of glass standards for sliding and bifold doors. If you would like to speak to one of the team about the glass, specification and standards for your project, call today on 01428 748255 or email

Glass inspection and the acceptable quality of vision

Q: How is the glass panel inspected?

A: Each glass panel is inspected from the inside, at a 2 – 3 metre distance in daylight, but not direct sunlight and from a 90o angle.

Q: What is an acceptable quality of vision?

A: Except for a 50cm band around the edge, known as the ‘edge zone’, the whole panel is inspected and is considered to have acceptable quality of vision if there are no obvious or clustered totally enclosed seeds, blisters, hairlines or blobs, minute embedded particles or fine scratches over 25mm in length.

Q: Is it possible for a pane of glass to be flawless?

A: Due to the manufacturing process of glass, additional treatments such as lamination or solar control and its inherent organic component, it’s incredibly rare for a pane of glass to be flawless.

Q: Is toughened, laminated, or coated glass inspected in the same way?

A: Yes. It’s important to note that glass panes which have been toughened, laminated, or coated are likely to have an increase in visible distortions, however this does not affect the quality and performance of the glass.

Q: Does solar-control coating affect the acceptable quality of vision?

A: Glass with a solar-control coating would be inspected in the same way and is subject to the same exacting standards. Coatings will increase the reflective nature of the glass and can therefore make visible distortions more noticeable, although there may be no increase in distortion incidences. 

Pure Glass ridge Light in New Malden home by Exact Architectural GLazing

Understanding the glass standards terminology 

Q: What is an edge zone on a pane of glass?

A: The edge zone is a 50cm band around the pane

Q: What is the viewing area of a pane of glass?

A: The viewing area of a pane of glass is defined as the area of glass between the edge zone and the critical area.

Q: How is a scratch defined on a pane of glass?

A: A scratch on a pane of glass is a surface flaw which has been produced by a hard object, creating a noticeable depression.

Q: What is a sleek on a pane of glass?

A: Unlike a scratch, a sleek is a fine surface imperfection without a noticeable depression.

Q: What are bubbles within a pane of glass?

A: Due to the manufacturing process, there may be small bubbles or pockets of trapped air which are partially or completely enclosed by glass.

Q: What does it mean if glass has an inclusion?

A: Occurring during manufacture, an inclusion refers to an insoluble piece of matter which can be either within or on the surface of the pane.

Q: What is a scar on a pane of glass?

A: A scar is a type of scratch which is visible and usually white in appearance due to the depth of depression.

Aluminium windows, sliding doors and roof glazing by Exact Architectural Glazing in Colchester

Meeting the acceptable glass quality standards

So, what does it mean for a pane of glass to meet the acceptable quality glass standards? With the above background in mind, the following minor imperfections in a pane of glass are deemed to be acceptable:

Viewing area defects

Based on the pane being inspected to the above method, the below imperfections are considered part of an acceptable quality pane of glass:

  • Scratches or streaks not visible during inspection
  • Seeds, blisters, hairlines, or blobs of less than 2.5mm
  • Bubbles or inclusions appearing in the viewing area less than 2.5mm in size and not closer than 200mm apart

We consider the pane to be unacceptable if there are white scars within a pane of glass.

Edge zone defects

It is considered acceptable for scratches, streaks, bubbles, inclusions, and scars to be within the edge zone, the 50cm band around the edge of the pane.

Condensation effects

Sealed units such as those used in sliding and bifold doors reduce the incidence of condensation, however, other factors can also cause the issue. Any formation of condensation on the pane does not affect the performance of the glass, nor does it indicate a defect in the glass.

Optical effects

Due to the manufacturing process of glass, optical effects can occur:

  • Roller wave distortion: a distortion as the glass is heat treated, which can produce a reflection
  • Brewster’s fingers: When high quality glass panes are multi-layered, a small rainbow effect can sometimes be seen.

Nickel sulphide inclusion

Glass that has been heat-soaked or strengthened undergoes a strengthening process during manufacturing to expose nickel sulphide (NiS) inclusions. It is impossible to guarantee the complete removal of NIS, therefore glass breakage due to NiS isn't covered with Warranty by any manufacturer.

Specifying the architectural glass for your project

Throughout your project with Exact, your dedicated Project Manager will work with you and the project team. During the specification stage, the glass specification for your project will be discussed and agreed to ensure the glazing will meet the visual and performance requirements. Find out more about our end-to-end project management approach here.

Contact us today on 01428 748255, email or book your Initial Project Consultation to discuss your next project.







Get in touch today to discuss your next project with one of the Exact team and find out why working with us could be the solution for you.




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