What makes a space sacred? What do we each bring to such places that sets them apart as special? What is appropriate behavior at such sites? What social and cultural roles do they play, and how inclusive or exclusive are those?

These are a few of the questions that come to mind while experiencing the installations by artists Jeffrey Mongrain and Nicholas Kripal within Trinity Episcopal Cathedral, Downtown. They're part of the Three Rivers Arts Festival and remain through Sunday.

They are also the kinds of questions that the ceramists have grappled with in recent years as they've sited work in places of worship in Europe and the United States.

Sacred spaces lend themselves to artistic intervention -- whether visual art, dance, music, drama, the setting for a novel -- since by nature they are psychologically, emotionally and physically removed from the everyday. At their most profound, such environments provide avenues to transcendence, and here, one might argue, is a point at which art and spirituality converge.

Mongrain lives in New York City and teaches at Hunter College of the City University of New York, and Kripal teaches at the Tyler School of Art, Temple University, Philadelphia.

They visited Trinity, among other churches, when in Pittsburgh in March and afterward submitted an art proposal to the clergy of Trinity Cathedral and the Arts Festival.

I say "experiencing" rather than "looking" at the four works because discovery is built into seeing these pieces. While searching them out, one is exposed to the architecture, symbology and/or beliefs of the place explored (a brief guide is also provided at the church's entries). The unmarked path may be internal as well as external.

Mongrain's "Sinai, Ararat, and Olivet," white incised mounds that represent three of the 26 most significant mountains of the Bible, is sited above the pulpit, from which the weekly sermon is delivered, and where the eyes of the congregation turn weekly.

It's the first time Mongrain has used mountains as imagery, and he created them proportionally. Mount Ararat, where Noah's Ark landed, is approximately 16,726 feet high, and Mount Sinai, where Moses received the Ten Commandments, 7,362 feet. The smallest, Mount Olivet at 2,859, is the most historically viable and has the most powerful symbolic presence as the place where Christ went after his betrayal and the location of the Olivet Discourse, a controversial commentary with potentially apocalyptic implications.

Discovering the stories is an enjoyable part of the research that goes into each project, Mongrain says.

He created "The Soul's Sea," a 10-foot span of "modeled black water," for The Cathedral Church of St. John the Divine, New York, after learning that during the 1892 excavation for the cathedral an underground spring filled the dug cavity. This he placed in front of the altar of the All Soul's Bay, which "honors the souls of the departed."

For "Synagogue Memorial," 11 clay pillow forms were suspended above seats left vacant, according to traditional Judaic Orthodox belief, to honor dead members of the Garnethill Synagogue in Glasgow, Scotland. Each contained a letter written by a friend or relative of the deceased.

Mongrain suspended a 54-inch high white clay plumb over a side passageway where 10 skeletons from the time of the Black Plague were discovered during 20th century remodeling of the Glasgow Cathedral.

The artists are sensitive to the nature of the spaces they interact with. While the majority of sites are potentially receptive, some have such tightly woven physical integrity that "almost any inclusion seems like a bruising," Mongrain says.

Kripal has also installed work in non-spiritual public locations, such as the Eastern State Penitentiary, Philadelphia. "There, the work related to the context of penal systems and rehabilitation," he writes in an e-mail.

Kripal's "Cathedral Labyrinths," complex terra cotta forms located in front of the main altar and inside the church's main entrance, "reference the historic placement of labyrinths within the floors of Gothic and Romanesque cathedrals for prayers and penitence. The labyrinth was seen as a symbol of the passage through life from birth to death to rebirth."

Placement is an important component of ritual, and the sculptures were put where they are because of the "significance of the ritual practiced at both sites," Kripal writes. "The one at the entrance is also adjacent to the columbarium, the resting place for the departed. The one in front of the altar is the site of the ritual of the mass and all of its transformative associations."

"I also think of these two pieces as 'stone flowers,' one in the act of opening up and one in the act of closing down -- the life cycle."

"They utilize the trefoil design found throughout Trinity Cathedral's architectural design and in many gothic or gothic revival churches," Kripal writes.

At a side altar, Kripal has installed "Reliquary: Double Helix" comprising hexagonal gothic modules that are grouped to reference a strand of DNA. Reminiscent of bundt pans, these rounded repetitive metallic-appearing units balance on a conceptual pivot between cool intellectual minimalism and stamped-out mass produced goods. Superimposing the building blocks of life upon a memorial altar that resembles an elaborate tomb construes a contemporary memento mori.

Implicit is the question of how we think about the remnant body in a spiritual space. "Also, the altar as a site for the sacrificial ritual/spiritual rebirth and, of course, Christ was divine but also human," Kripal writes.

To those, of faith or virulently without, who feel art has no place in spiritual spaces, Mongrain says simply "Look around." Certainly much of the history of art and architecture is entwined with that of religious and other belief systems.

Consider the cavernous gothic cathedrals, sensually enlivened by the washes of color flowing onto the stone floors from their myriad jewel-like stained glass windows. At the other end of the secular spectrum is Robert Smithson's "Spiral Jetty," which, by the addition of an oversized archetypal shape, transformed the Great Salt Lake from mundane landscape to sacred humanist shrine.

Mongrain and Kripal offer the experience, as ephemeral as life itself, to visit a space apart. It's a journey worth taking.

The Cathedral is at 328 Sixth Ave., Downtown. Because of grounds renovation, entry is through the adjacent First Presbyterian Church gates. Hours are 8 a.m. to 5 p.m. today and tomorrow, 9 a.m. to 1:30 p.m. Saturday, and 7 a.m. to 1:30 p.m. Sunday.