THE COST OF A CLEAN LINE

What a curved-glass facade taught me, and what an owner should ask before approving one

The idea was simple to say and hard to draw: a curved glass facade, concave toward the outside, made of tall vertical panes joined by the thinnest possible silicone joints, so that from the street you could look straight through it to the ambulatory behind. Maximum transparency, minimal joints, nothing in the way. A clean line.

I worked on that facade as a project director, alongside a team of architects, facade consultants, mechanical engineers and manufacturers, and I knew very little about curved glass when I started. What follows is what I learned from them, in the order it surprised me, and the questions I would now ask before anyone approves a facade like this one.

Curved Glass Is Not Flat Glass

I had assumed curved glass was flat glass with a shape. It is a different product, with its own rules.

To bend glass, you heat it in a furnace above the temperature at which it is normally annealed, and that heat erases the stresses left by cutting and grinding. What happens next depends on how fast it cools. Cooled very slowly, it is heat-annealed: relaxed, with no residual stress. Cooled faster, it becomes heat-strengthened. Cooled fastest, it is tempered, and it will break into very small pieces where the other two break into larger ones.

That choice is not a technicality. It changes how strong the glass is, how it breaks, and how it looks. Heat-treated glass can show anisotropy, a faint iridescent pattern visible from certain angles, and curved panes can show distortion near their edges. Some manufacturers told us that annealed curved glass gives the best optical quality and can still take coatings and frits. Others pointed out that a curved pane is stiffer than a flat one, which is why it can be fixed only along its curved edges, at the top and the bottom.

Even the geometry was not ours to decide alone. The curvature had to be adjusted to what the manufacturers we consulted could make and what could be transported, which meant that the radius on the drawing was, in the end, a negotiation.

Transparency Is Never Free

The design asked for maximum transparency, and every layer of glass takes some of it away. A double-glazed unit and a triple-glazed unit do not transmit the same light, they do not perform the same, and they do not need the same building behind them. We compared them, with different interlayers, and each option moved a different number.

There was also light protection. The ambulatory behind the glass was not designed to hold sensitive elements, but UV filtering was still necessary, and it competes with transparency for the same square meter of glass. Coatings help, and each manufacturer finds a different balance between the targets: transparency, thermal performance, solar control and UV. The glass has to meet all of them at once.

And then the building has to cope with whatever glass we choose. Thermal comfort in a space like this is not negotiable, and neither is the absence of condensation: a very tall volume behind a large surface of glass will condense water unless it is designed not to. The mechanical services were sized according to the type of glass. Warm air is released at the floor through a slot that follows the curve of the glazing, and at the top, diffusers and circulation fans move warm air through the space. None of this appears in the elevation, and all of it is a consequence of one decision about glass. I had not expected to learn so much about air in a project about transparency.

The Lens

A concave surface can focus the light it reflects. It is basic optics, and a well-documented risk of concave glass facades: under the right angle of the sun, reflected light can concentrate on a street, a terrace or a landscape and heat it far beyond what anyone expects. We understood it as a minor risk for our design, and I still think that is fair. But it depends on geometry, orientation and the position of the sun, and it is far cheaper to study on a drawing than to correct on a building.

What the Owner Inherits

The architect's relationship with a facade ends, more or less, at handover. The owner's does not. A glass facade has to be cleaned from the outside and the inside, safely, for decades. It has to resist impact and vandalism, and it can break for reasons nobody planned for, from an unlucky thermal stress to a stone. And because curved glass is made to measure, a broken pane cannot be replaced from a shelf.

For this client, security, maintenance and cleaning were as important as the image. We presented an exterior cleaning strategy. And the client did something I think every owner should do: they visited several buildings with curved glass of similar dimensions. Those visits were very valuable. They gave the client first-hand knowledge of how this kind of glazing behaves over the years: how panes can break, why spares are kept in storage, and what replacing one involves. Replacement, we learned, is not only a matter of having the glass: the labor and the equipment it requires are very expensive.

There is a quieter issue too: coating technology never stops advancing. If a pane has to be replaced years from now, the coating available then may not be the same as the original, and the new glass may look slightly different from its neighbors. It is something we learned along the way, and it has to be part of the maintenance strategy from the start.

What a Review Is For

During design development, the client asked for an independent review of the facade. It was a technically complex piece, and they wanted someone with deep experience to give the design solidity, to suggest improvements and to reduce risk: to safety, to maintenance, to the integrity and the longevity of the facade, and to the collection inside the building.

It was a valuable exercise. We adjusted the curvature, and the glass composition continued to be refined with the manufacturers. What surprised the client most was longevity, and the comparison between double and triple glazing.

It also taught me something about reviews in general. Their value is not only in the document. A written report gives you conclusions, and a conversation gives you the reasoning behind them: the chance to ask why, to test an alternative, to explain a constraint the reviewer could not have known about. If I were setting up a review like this again, I would plan for that conversation from the start, with a follow-up meeting between the reviewers and the design team.

Who to Ask What

One thing I learned is that the questions have three different addressees.

To the manufacturer, I would give the targets first: light transmission, thermal performance, solar heat gain, UV. Then I would ask which glass they propose, with what composition and which coatings; how it behaves under temperature changes and how long it lasts; how it is fabricated and transported, and what are the largest and smallest curvatures and sizes they can make; and what happens when a pane breaks, how long it takes to replace, with what equipment, and how many spares should be kept.

To the facade consultant, I would ask how the glass integrates into the structure, and which coatings give the design the thermal, structural and optical stability it needs. These are details an architect has often not had to resolve, and they are why a consultant earns an early seat at the table.

And the owner has questions to answer for themselves: the minimum life they expect from the facade, the spare panes they will store, how often it will be cleaned, how it must resist vandalism and thermal breakage. And, if there is a collection behind the glass, the conservation standard that sets the maximum UV transmittance, because that number will shape the glass more than the geometry does.

What I Take With Me

The earlier manufacturers, facade consultants, mechanical engineers and reviewers are in the conversation, the more options stay open, and the more a clean line can be defended with knowledge and not only with intention. A facade like this is never the work of the architect alone. It is the work of everyone

who knows something the architect does not.

The cost of a clean line is not paid at the drawing board. It is paid in condensation that must never happen, in cleaning cycles, in the spare panel in storage and the specialist crew needed to replace a single pane, and it is paid by whoever owns the building. The best moment to talk about that price is before the line is drawn.

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Alicia Hernanz is the founder of Anantu.Studio, an independent consultancy for museums and cultural institutions. She spent over a decade with leading European architecture practices, including five years working with Barozzi Veiga directing the design and delivery of a major expansion at the Art Institute of Chicago.