Glass · Identification
Is this low iron glass, or ordinary clear float?
Sight along the edge. Ordinary float is green there; low iron is close to colourless. But do not trust your eye on a single pane — hold a known offcut of ordinary float against it, edge to edge, and the difference stops being subtle. Everything below is why that comparison matters more than the test itself.
Why the edge is where it shows
The mechanism is worth two sentences, because it explains every practical rule further down this page.
Ordinary clear float carries iron oxide left over from the sand and the batch. Iron in its ferrous state absorbs at the red end of the spectrum, and what survives is the familiar green. Low iron glass — sold as Optiwhite, Diamant, Starphire, UltraClear and half a dozen other trade names — is made from purer raw materials with roughly a tenth of the iron, so far less is absorbed and the glass stays close to neutral.
The colour is an absorption effect, so it depends on how far the light travels through the glass. Face-on through a 4 mm pane that path is 4 mm and you see almost nothing. Along the edge of the same pane the path is the whole width of the sheet, so the green concentrates and becomes obvious. That is the entire trick: you are not looking at the edge because the edge is different, you are looking at it because it is the longest path through the glass you can find.
Approximate, and on a screen rather than in your hand. The point of the pair is the comparison, not the exact hue — apart it is a guess, together it is obvious.
The checks, in order
The first one is not a test, and skipping it is why people argue about this on site.
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Get a reference offcut in your other hand
The green of ordinary float is a soft, desaturated bottle colour, not the vivid green anybody pictures. On its own a pane looks broadly clear and your eye adapts to it within seconds. Against a known offcut of ordinary float of similar thickness, the difference is immediate and nobody argues.
Keep a small piece of each on the rack, labelled, for exactly this. It costs nothing and it settles a question that otherwise ends with somebody phoning the supplier.
Every check below assumes a reference. Without one, all you can honestly say is "probably".
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Sight along the edge, against white
Hold both pieces edge-up over a sheet of white paper or against a white wall, in daylight rather than under warm artificial light, which pushes everything yellow and hides the green. Look down the long edge. Ordinary float reads clearly green; low iron reads white, grey or faintly blue.
This is the whole test. Everything else on this page is either a confirmation or a reason to bother.
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The white paper test, face-on
Lay each pane flat on white paper and look straight down through it. Ordinary float tints the white a faint green; low iron leaves it white. This works well from about 10 mm upwards and gets unreliable on thin glass, because face-on the light path is only the thickness.
Convincing for a client, who can see it without knowing anything about glass. Weak on thin panes.
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Stack it, if it is thin
If you only have 4 or 6 mm and the edge is inconclusive, stack several offcuts of the same glass face to face and look down through the stack. You are artificially lengthening the light path, and the colour that was invisible in one pane accumulates into something you can judge.
A workaround, not a standard method. Useful when the edges are chipped, dirty or painted and you cannot sight along them.
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Ask the supplier, with the batch in front of you
Low iron is not marked on the glass. Toughened glass carries a safety mark under EN 12150-1, but nothing on a pane declares its iron content, so there is no equivalent of the kitemark to look for. What exists is paperwork: the delivery note, the order, and the manufacturer's product name.
The only documentary answer. If the job depends on it, the place to settle it is the order, before the glass is cut.
Where it is worth the money, and where nobody will ever see it
The colour builds with the light path, so the answer is mostly about thickness and about what is behind the glass.
| Job | Does the green show? |
|---|---|
| 4–6 mm window | No. Face-on through a thin pane there is nothing to see, and low iron here is money spent on something nobody will notice. |
| Splashback, back-painted white | Yes, badly. The light goes through the glass, off the white paint and back through the glass, so the path is doubled and a white splashback comes out pale green. |
| Balustrade, 17.5 or 21.5 mm laminated | Yes, at every edge, and the edges are what people stand next to. This is the classic reason to specify low iron. |
| Glass floor or tread | Yes. Thick, laminated, and normally viewed at an angle, which lengthens the path further. |
| Shopfront and display cabinets | Yes, on anything pale or white behind the glass. Merchandise colour is the whole point of the fitting. |
| Mirrors and back-painted colours | Yes, and doubled for the same reason as a splashback. Pale colours shift, dark ones hide it. |
| Solar and photovoltaic covers | Yes, and it is measurable rather than aesthetic — more light through the cover is more output. |
Light transmission is a little higher for low iron at the same thickness, and the gap grows with thickness because that is where ordinary float loses the most. Take the figures from the manufacturer's data sheet for the actual product rather than from a rule of thumb: they differ between makers and between coated and uncoated glass, and on a solar job the difference is the specification.
What low iron is not
It is not a safety grade. It is a composition change made for optical reasons, and it says nothing about impact performance. Low iron glass is annealed, toughened or laminated exactly like ordinary float, follows the same thickness rules for wind load and for balustrades, and takes the same standards and markings when it is turned into safety glass. If a location calls for toughened or laminated, low iron does not answer that question — it is a separate one.
It is also not "extra clear" as a loose description. Suppliers use a lot of words for pale glass, some of which mean genuine low iron and some of which mean ordinary float sold well. If the job depends on it, get the manufacturer's product name on the order, not an adjective.