In a previous post, we noted that some contemporary architects are searching for an ultimate transparency, aiming for “bigger, wider, clearer” glass wall . This trend is not only present in Sci-Fi but also in many recent real projects, the bigger the better. This trend is strong and more than transparent, it seems that the ultimate goal set for glass is to disappear while keeping its capacity to define a physical limit.
FIG : Oversize window Star Wars / Airbus Prototype : the interior face of the plane is a digital display showing the outside
Actually the “disappearance” of the appearance of glass is already possible. This quasi-perfect transparency or hyper transparency can be reached using extraclear glass with a non-reflective coating .
For soda lime glass, which is the most prevalent glass type used in architecture, the iron content of the sand which is used, affects its clarity and its hue. The higher the iron content, the greener and darker the glass gets. To illustrate this phenomenon, here are two pictures.
Fig: saturated Transparency /auto multi-reflection of mirrors
The first one was shot perpendicularly to a row vertical glass stiffener in the lobby of a tower in Hong Kong. It is quite amazing to see how the people in the far distance disappear behind the glass, as the glass gets greener. When thick glass is used, the composition usually turns greenish when using typical float glass. We are witnessing some kind of saturation of transparency, realizing that glass is not fully transparent. I find this fascinating, as well as the dark area in the center of the image, what is this dark area made of ?
The second picture shows two mirrors facing each others, the mirrors are made of regular float glass with a silver coating, bathroom mirrors. As the reflections multiply, one can clearly see the “greenness” appearing more and more, reflection after reflection the glass reflection is acquiring a bit more of the green hue of the original panes. Again the center of the image gets darker into some kind of dark field.
Every major glass producer has now its own extra clear glass float line, which is also refered as low iron glass, for example , the Diamant for SSG, optiwhite for PPG, ultra clear for Guardian …., Actually the amount of iron in a glass production isn’t always so easy to control, particularly when the origins of the sand (and its iron content) vary… Even among extra clear glass products, some very slight variation of hue can be noticed when looking very carefully.
Some glass compositions like the Borosilicate glass are naturally clear and produced some optically pure glass . This type of glass was originally developed by Schott in Europe and then by Corning in US. In architecture Borosilicate glass find its use in fire application mostly due to its high fusion point and in some cases when the architect looks for very clear glass.
The Louvre Pyramid was one of the first architectural projects made with extra clear soda lime glass, with a clarity that could be comparable to a borosilicate glass, which greatly improves its inherent transparency .
Fig : AR coating, Destructive wavelength superposition with a AR coating / AR coated glass used for a vitrine in a museum or for PV cell assembly
The second property of glass, after transparency, is its extreme smoothness and therefore, its reflectivity. In fact, glass is mostly perceived through its reflectivity. Depending of the viewing angles, the transparency can even be obstructed fully, revealing just an plan of reflection.
This subject is quite sensitive in museum where glass protects rare artwork. Indeed the visitors tend to see more the reflected environment than the art piece. More and more, the glass used for this type of application, is coated with an anti-reflective coating (AR)
The AR coating are made of reflecting particles (usually metallic) which are deposited on the face of glass, to reflect some targeted wavelength of the incident light. One layer for one specific wavelength. The thicknesses of each layers are chosen so that the reflected wave of the incident wave creates a destructive superposition which actually cancels out the reflection . This is done for each layer of the AR coating, which targets a specific wavelength. Some AR coating can go up to 200 coats for as many wavelengths for optic lens of spy plane or an astronomy cameras, which need to be as transparent as possible. (Therefore with no reflection)
This type of coating brings the reflection of glass down from 8% for a normal glass to 1% or less, enhancing transparency to a point where it is actually difficult to perceive that there is a glass panel.
Light transmission is also enhanced which explain why this type of glass are mainly used by the solar cell industry to enhance sun gathering efficiency through higher transparency.
Even if cost and conditions of utilization limits the possibility of use, I am quite surprised to see how few projects can be found with such glass, when the aim for transparency is on the other hand so strong .
Fig: One North Waker cable net lobby entrance Chicago using amiran glass
One of the first project with AR coated glass is the One North Wacker lobby in Chicago This glass wall was realized by ASI inc (bought by Enclos) It illustrates very well this aim for the disappearance of Glass. The combination of a cablenet structure, only composed of stretched cables within a big frame support of glass panels, made the Amiran from Schott (1) which propose a very pure glass, extra clear, with an antireflective coating, make it hard to imagine that there is a glass wall in far distance. Although, the glass panes are quite small and one can still notice the cables of the nets and all the connections at each panes corners.
In fact, it is a different approach from the recent trend which is to not only use very large panels, up to 3×15 m, but also to have all mechanical “things”, ei metal connection or support, removed as much as possible. Only glass, transparent glue, transparent joint, highly polished edges. This is typically the “all in glass” work developed for the Apple store, with every year a new record of the length of glass : 8m, 10m, 12m, 14 m now 15 m or even 18m …
Fig: Apple Store Turku Istanbul,Transparent glass box (Left) with reflection (center and right)
Unfortunately, may be obsessed by sizes of its glass panes, Apple has never used (to my knowledge) non reflective glass in its store, I wonder why ? (too many laminates certainly which kill the AR coating effect? not expansive enough ? or slight irisation on the edge which is not pure enough)
May be transparence is not the only aspect that the marketing is looking for. In fact the glass needs to be visible, to stay the symbolic vehicle of the future, which, as a material, it has always represented. This is why it is important to associate your image with glass, same as “titanium” sounds cool and futuristic, or wood for the store furnitures is the eco-friendly material …..
At the end, it is may be as well that very few use anti-reflective glass, if the disappearance of glass, even symbolically, can be seen as another great performance, it can turn also into a great loss, reducing the aesthetic potential of this material, to some slighty too purist trip to be sane.
Top AR glass/normal glass/ mirror coated glass Bot : reflective logo on AR coated glass / invisible from the inside/ “See article predicting transparency” for more information
Unless, one imagines to use AR-coating combined with others coatings to find some other means to enhance transparency. This is what Etienne Sandre Chardonnal and I have done with this principle which uses an AR coated glass and a reflective coating applied through a serigraphic screen to make a logo , which only appears in reflection.
Many others ways can be explored to tame or play with this notion of transparency, using blur, parallax on opposing pattern , variable reflection coating … ect Glass doesn’t have to disappear to exist, if it disappears it should only be to reappear in a more exciting form.