This is a carousel. Use Next and Previous buttons to navigate

San Francisco's skyline tends to change in vertical bursts, and in five years we could be looking at nearly a dozen new thin-skinned boxes wrapped in sheets of green and blue. Some promise to be better than others. The cumulative effect could erode the physical presence that makes this city's downtown distinct.

The issue is not that buildings clad in stone are morally superior to buildings clad in glass. It's that San Francisco's sense of place is tied to its earthy warmth, the juxtaposition of steep hills and shifting waters. Too many glass towers clumped too closely together would set a much different tone, cosmopolitan but also generic.

At least 10 major buildings now in the works are conceived as being nearly all glass; some are under construction while others are awaiting financing or still being reviewed. The city needs to shift focus to the overall impact of an architectural trend rather than try to make the best of each tower on its own - and nudge developers and architects to introduce the material and depth that is satisfying not just at first glance, but year after year.

The popularity of glass towers isn't new here, where a half-dozen crystalline skyscrapers debuted as the recession took hold in 2008 and 2009. Nor is it unique to San Francisco. "Glass is the new brick," architecture critic Paul Goldberger mused in a 2006 essay on Manhattan's residential scene.

But as the Bay Area economy shakes off its torpor, see-through forms continue to be the look of choice. At least three approved residential towers have been retooled for the new decade, with masonry elements replaced by glass and rectangular forms replaced by presumably jazzier ones. Even more striking, two existing towers are slated for architectural makeovers, with their original skins of concrete being stripped away in favor of you-know-what.

One transformation is already in process at Third and Folsom streets, where the steel bones of a 1965 office building stand exposed as workers do seismic work before adding a clear skin. The other involves the much sturdier AAA tower at 100 Van Ness, two blocks south of City Hall; the structural core of the 29-story tower from 1972 will stay intact, but the space inside will become apartments, with a glass veil to "bring something new and fresh to Van Ness," in the words of architect Chris Pemberton of the firm Solomon Cordwell Buenz.

The two buildings being altered will improve as a result. The 680 Folsom mid-rise in its original form was a crude chunk that emphasized how much of a backwater that part of town once was. As for 100 Van Ness, it personifies the refrigerator-box modernism that caused San Franciscans to demand more sensitive architecture in the 1970s and '80s.

But if 680 Folsom makes the best of a drab situation, the stakes at 100 Van Ness are much higher.

The project was presented Thursday to the Planning Commission, which responded favorably to the aesthetic overhaul of a building that developer Oz Erickson characterized as "Stalinist." Ominously, though, no rendering was shown from the most crucial perspective of all: Civic Center Plaza. Here, in America's most imposing ensemble of City Beautiful-era buildings outside of Washington, D.C., a glass box will hover far above the scene.

Done right, the contrast can be an elegant one that marks a clean break between Then and Now. But it also could be jarring, an alien presence behind the columned granite and gold-leaf dome of City Hall.

Even as the commission gave its blessing, city planners and commissioners emphasized that the color of the glass is still in flux, as are details that would give the two-dimensional form more depth. The scrutiny is deserved; what counts in this location isn't a cool silhouette that catches the eye of potential residents, but a design that can both strike a fresh tone and complement its neighbors.

There's a different issue along Folsom Street in the heart of the high-rise districts planned for Rincon Hill and the blocks directly to the north: not whether a lone glass tower will be disruptive, but whether glass will overwhelm everything else.

Two all-glass towers already are in place, the 43- and 38-story Infinity complex. Now two more are on the way across the street with the same dimensions. They also have the same lead designer, Arquitectonica, a Miami firm.

While the Infinity consists of two static deep-green clover shapes, the 201 Folsom complex is depicted with a silvery blue sheen and a more dynamic form, each sharp oval tower pivoting slightly toward the other. Still, that's a lot of glass in an area where large development sites abound and at least one other all-glass shaft is planned a block away.

And an urban district that shimmers will be different than ones where the mood is set by wood or brick or stone or concrete. Those elements are tactile by nature. They emerge from the ground, in a sense, and with time they settle into a city's landscape whatever the inherent merits of the design.

By contrast, "glass has always been considered to be the epitome of contemporary materiality," says Fred Clarke of Pelli Clarke Pelli, the Connecticut firm designing the Transbay Tower that would rise 1,070 feet at the corner of First and Mission streets. "It allows powerful abstract statements that can be the perfect complement to old masonry buildings."

That was the perception here when the Crown Zellerbach building opened in 1959 at Market and Bush streets. The 20-story building with its International Style skin of glass and steel was hailed as an invigorating addition to a downtown that had seen almost no new construction since the Depression.

But when the exception becomes the rule, a district becomes weightless. The pedestrian realm of sidewalks and alleys and shop fronts becomes an afterthought, the pedestal for the bauble on display up above.

Even architects who make a specialty of such towers see the danger.

"This is a city of depth and variety of materials," says Clarke, whose firm's 560 Mission St. frames glass in suave metal in such a way that the 2002 high-rise is a comfortable fit amid lower masonry structures. "How a building meets the ground is an appropriate concern."

This is a concern of city planners as well.

Several towers in the works have been massaged so that the lower floors have a more solid horizontal look, as if to make them seem rooted. There's also a push for balconies, or additional setbacks and folds, anything to add layers of interest in the air.

"The current trend is for very taut glass towers," says John Rahaim, the city's planning director. "Maybe I'm conservative, but I don't think that a preponderance of taut glass works well in the city."

This isn't to say that San Francisco should return to the planner-driven architecture of the 1980s and '90s, which kept the worst from happening but also squelched creativity. Nor are all glass towers alike.

Even so, developers, architects and planners all share a responsibility to the setting in which they leave their mark. San Francisco's downtown isn't perfect, but it has far more character than most large American cities. If a wave of cold glass boxes dilutes this, we'll have squandered something special.

John King is The San Francisco Chronicle's urban design critic. E-mail: jking@sfchronicle.com Twitter: @johnkingsfchron

John King is The San Francisco Chronicle's urban design critic, taking stock of everything from Salesforce Tower to sea level rise and how the pandemic is redefining public space. A two-time Pulitzer Prize finalist and author of two books on San Francisco architecture, King joined The Chronicle in 1992 and covered City Hall before creating his current post. He is an honorary member of the American Society of Landscape Architects.