Curved glass guidance

Curved glazing is possibly the most optically and dimensionally challenging glass. Many projects are making use of the advancing curved glass processing technologies which are allowing tighter radii, greater girths and greater overall dimensions. However, the optical and dimensional challenges are bespoke to curved glass and therefore the many standards for flat glass do not apply. Until the recent publication of BS ISO 11485 Part 2 we had no guidance from European standards on the quality to expect.

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BS ISO 11485 Part 2, published at the end of 2012, now offers the industry an alternative set of quality criteria to the American ASTM C 1464-06 (reapproved 2011) for cylindrical (radius) bent float glass (neither standard covers more complex forming including cone form or other simply describable geometries). Whilst both standards require the defects to be assessed from the identical distance of 2m, the dimensional quality criteria is not so easily comparable. Differences between combinations of glass thickness, length, girth and their individual ranges for allowable dimensional deviations, makes it necessary to convert from one guideline to another before an informed comparison can be made.

Our findings have generally shown that the ASTM is more onerous for dimensional tolerances, but that this is likely to be due to the BS relaxing allowances for thinner glazing, greater girths and greater lengths. However, in some instances (e.g. where the girth or glass length is small) the ASTM can be less onerous. An example of one of our conversion graphs is shown below to facilitate easier comparison for girth dimensional criteria.

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Where curved glass is employed for a project, it becomes sensible to derive the ASTM and BS quality standards using the glazing specific length, girth and thickness. This gives the design team opportunity to understand what they are to expect from their glass size and time to accommodate the dimensional tolerances in the system design. It is good practice to make all parties aware of the expected standards and so ensure the glass processor, facade contractor/systems designer, architect and client are all aware of the agreed criteria that is being worked to prior to moving to manufacture.