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Visual inspection of glass

• Introduction
• Published guidance
• Specification

Introduction

Most specifiers are blissfully unaware of the conflicting standards and guidance for assessing the visual quality of glass until faced with a dispute and, when the visual quality of glass is questioned, the first item of dispute is generally the distance at which the glass should be viewed when making quality assessments.

Viewing distances vary between 1m and 4.5m - see Published guidance below. There is an obvious perception within the industry that a shorter viewing distance must result in more stringent quality criteria - "shorter is better" - but this is not, necessarily, the case.

CWCT Technical Note 35 (see below) makes the excellent statement ...

The viewing distance and the associated defects to be viewed are inextricably linked; a specifier or an observer should not arbitrarily use the viewing distance from one Standard and the viewing criteria from another.

... but also adds ...

The specifier should not specify viewing criteria or distances more stringent than those given in the BS EN Standards for the glass in question as the glass supplier will be unable to impose tighter standards except possibly with regard to overall bow, local bow and roller wave in heat strengthened or toughened glass.

This fails, unfortunately, to help the specifier with selecting the appropriate standard to be used when there is the potential for conflict, e.g. which standard should be quoted for a double glazed unit consisting of panes of both annealed glass and laminated glass and when one (or both) of those panes is coated? It also fails to take account of the potential problems when glass manufacturers may be working to different standards - EN 1096 compared with Hadamar, for example.

Published guidance

British Standards

These three standards have been chosen solely to highlight the discrepancies between the stated viewing distances. Any discrepancies (between the standards) as to what constitutes a visual defect, and the pass/fail criteria for these defects, is not discussed further. A more comprehensive list of the standards applicable to glass is provided in CWCT TN35 (see below), Table 3.

• BS EN 572-2:2004 : Glass in building. Basic soda lime silicate glass products. Part 2: Float glass. Clause 5.2.1 (and figure 3) states a viewing distance of 4.5m.
• BS EN 1096-1:1999 : Glass in building. Coated glass. Definitions and classification. Clause 7.3.1 states that the minimum viewing distance should be 3m.
• BS EN ISO 12543-6:1998 : Glass in building. Laminated glass and laminated safety glass. Part 6: Appearance. Clause 9 states a viewing distance of 2m.

Trade organisations

• CWCT (Centre for Window and Cladding Technology) : Technical Note TN35. Assessing the appearance of glass. This is probably the best starting-point for those with access to this document. In addition to listing the relevant British Standards, reference is also made to the GGF Guidelines and the Hadamar Guidelines.
• GGF (Glass and Glazing Federation) : Quality of Vision - double glazing. This document, publicly-available to download from this page of the GGF website, states a viewing distance of 2m in general and 3m for "toughened, laminated, or coated glasses" (note that this conflicts with BS EN ISO 12543-6). The acceptance criteria are described in far less detail than in the British Standards referred to above.
• Institute of the Glazing Trade for Glazing Technology and Window Manufacture (Germany) : Guideline to Assess the Visible Quality of Glass in Buildings. June 2009. This document (often referred to as the "Hadamar" standard) states, in clause 2, that the viewing distance should be 1m.

Glass suppliers/manufacturers

This is a representative selection of guidance readily-available on the internet but must not be taken as a comprehensive review of the information available from the glass supply industry.

• Eckelt Glas Tolerances Handbook : Refer particularly to Section 4.3, starting on Page 29 (29 of 85), where the viewing distance is stated as 1m, or 1.5m for cast glass.
• Technical specification for Arcon coatings : An incredibly detailed document, cross-referred to (BS) EN 1096-1. The viewing distance is therefore 3m.
• Isolar Glas Technical Information : A translation into English, by Interpane, of the "Hadamar" standard (see above).
• Guardian SunGuard Quality Inspection Guidelines : Refers to the European Standard EN 1096 and, hence, a viewing distance of 3m.

Specification

National Building Specification (NBS)

The NBS relies on three essentially similar clauses - H11/737, H13/610, and L40/155 - to describe "Glass generally". All three clauses include the phrase "Glass quality: Clean and free from obvious scratches, bubbles, cracks, ripplings, dimples and other defects." but without describing, explicitly, the criteria to be used to assess the term "obvious", except by reference to criteria in the individual product standards.

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