Glassary - N

Negative Image:
a term used in sand blasting and acid etching. It refers to the area of glass that has been eroded by the sand or acid.

Newton's rings:
a fault occasionally found in insulating glass units caused by exceeding the maximum recommended size. If a large insulating glass unit is made with a narrow cavity it is possible that the two panes of glass can deflect and touch, causing a cold bridge. This can lead to a ring of condensation forming at this point.

Nickel sulphide inclusion:
small impurities in the glass that can cause spontaneous fracture of toughened glass some time after toughening. Enquiries about this problem should be addressed to Pilkington Technical Advisory Service on 01744 692000.

Noise attenuation:
an alternative term for sound insulation.

Noise control:
see acoustic properties.

Non-combustible:
see reaction to fire.

Non-insulating glass:
a fire resistant glass which gives integrity for at least 30 minutes, when glazed in a suitable screen, but which does not give insulation for 30 minutes. Pilkington Pyroshield™ is an example.

Non-loadbearing element:
an element of construction, which plays no part in supporting the building structure or part of the structure. Glazing is very rarely used in a structural capacity.