Glass types

Low-iron (ultra-clear) glass

Ordinary "clear" glass is faintly green, an artifact of the iron in the sand it is made from. You usually do not notice it until you look at a thick piece edge-on, stack several sheets, or set clear glass against bright white. Low-iron glass strips out most of that iron to give a genuinely colorless, water-clear result, for a price.

5 min read All levels High clarity

Why ordinary glass is green

Float glass is mostly melted sand (silica). Natural sand always contains a little iron oxide, and iron absorbs light unevenly, taking out more of the red and blue and leaving green to pass through. In a single thin pane the effect is subtle, but it adds up with thickness and with every additional sheet. That is why the edge of a piece of clear glass looks distinctly green, and why a thick shower panel or a stack of shelves takes on a green cast. See clear float glass for how the base product is made.

Standard clear float green edge VLT ~83–89% (thicker = greener) Low-iron (ultra-clear) near-colorless VLT ~91%+ (stays neutral)
Same thickness, different iron content. The green cast of standard glass is caused by iron oxide absorbing part of the spectrum. Low-iron glass uses purer raw materials, so it transmits slightly more light and, more noticeably, stays neutral instead of going green as it gets thicker. Light-transmittance figures vary by thickness and product.

What low-iron glass is

Low-iron glass (also sold as ultra-clear, extra-clear, or optically clear glass) is float glass made with sand and batch materials that have had most of the iron removed. The result:

  • A neutral, colorless appearance instead of a green tint, the difference is dramatic edge-on, in thick glass, and when several lites are stacked or laminated together.
  • Higher visible light transmittance. Clear glass transmits roughly 83 to 90 percent depending on thickness; low-iron is typically about 91 percent or more, and the gap widens as the glass gets thicker.
  • Higher total solar transmittance, because removing the iron also lets more solar energy through (see the trade-off below).

Common brand names include Starphire® (Vitro), Optiwhite™ (Pilkington / NSG), Diamant® and UltraWhite (Saint-Gobain), Guardian UltraClear™, and AGC Clearvision®. They are all low-iron float glass; the naming differs by manufacturer.

The trade-off: it lets more solar heat in
Iron does not just affect color, it also blocks part of the sun's energy. Removing it raises the glass's solar heat gain coefficient (SHGC). That is an advantage for passive-solar designs, greenhouses, and the cover glass on solar panels, where you want maximum energy through. It can be a disadvantage on a sun-facing facade in a cooling-dominated climate, where higher solar gain means higher air-conditioning load. Low-iron is frequently paired with a Low-E coating to get the clarity without giving up solar control.

Where it is worth the premium

Low-iron glass costs more than standard clear (often on the order of 10 to 30 percent more, depending on market and product), so it is specified where clarity genuinely matters:

ApplicationWhy low-iron
Frameless shower glassThick (10 to 12 mm) glass next to white tile, where green is obvious
Glass railings, partitions, balustradesLarge, often thick panels viewed edge-on
Storefronts & display casesTrue color rendering of products behind the glass
Furniture, table tops, shelvingThick edges and stacked sheets exaggerate green
MirrorsA truer, less green reflection
Solar / PV cover glass, greenhousesMaximum light and solar energy transmission
Museum / picture glassColorless viewing, often with an anti-reflective coating

For most ordinary windows, standard clear glass is perfectly fine, the green is invisible at typical thicknesses behind a frame. The case for low-iron grows with thickness, with exposed edges, and wherever color accuracy is part of the design.

It is still the same glass otherwise
Low-iron glass tempers, laminates, and coats exactly like standard float glass and can be turned into safety glass, IGUs, or mirrors the same way. The only differences are clarity, a small bump in light and solar transmittance, and cost. It is a substitution, not a different process.
Disclaimer: Light-transmittance values, solar performance, and price premiums vary by thickness, product, and manufacturer; brand names are illustrative. Verify the specific visible-light and solar figures against the manufacturer's current data for the exact product and thickness before specifying.