Teresa Margolles is standing in a warehouse on the Thames estuary, surrounded by large boxes marked “Frágil”. They have come from Mexico, holding the face masks of 370 transgender, non-binary and gender nonconforming people. Cast in white plaster, each bears traces of the person on whom it was moulded: a bright smear of lipstick here, a false eyelash there; even, in one case, half an eyebrow. Each has a number and a name – Leila, Milla, Maga, Bruno. One by one, the casts are released from their packing and gently placed on a podium, concave side up, for Margolles to photograph. Dressed head to toe in her trademark black, she works with the respectful precision of the forensic pathologist she once was, beckoning me over to inspect the latest image on her camera. It shows the concave mask plumped back into the face of participant number 144, whose name is Paulina. “Every face has a story attachecd,” says the 61-year-old Mexican artist. This painstaking process of documentation is a by-product of the project that Margolles has come to London to complete. She is shortly to become the 15th artist to have an installation on the fourth plinth in Trafalgar Square. The 370 faces from Mexico will be joined by another 370 from London (revised down from the original total of 850), displayed inside-out in a huge cube of what their creator describes as “life masks”. The project was inspired by the Mesoamerican tradition of tzompantli – racks that were used to display the skulls of sacrifice victims or prisoners of war. Margolles is not worried about them decomposing in their two-year exposure to London’s rain and pigeon droppings. That’s the whole point, she says. “They will fade and transform. It’s a natural process. What interests me, and the reason why I’m not displaying the outside of the casts, is because it would be breaking the soul of it, which is the face of the person. Like this, each cast will react to the elements in its own way, according to the organic materials left on the mask.” If the idea of 740 faces disintegrating in the rain sounds a bit ghoulish for a project designed as a celebration of public art, in a place where people from all over the world come to eat ice-creams and take selfies beside fountains and red doubledecker buses, that’s par for the course for the artist. Compared with much of her work, Mil Veces un Instante (A Thousand Times in an Instant) will be a celebratory monument, for the simple reason that it is based on the living rather than the dead. Over nearly four decades, Margolles has made murder her material. Her studio is attached to a morgue in Mexico City, where she often works with fluids from the corpses that are brought in from the streets. Mil Veces un Instante is a memorial to one such victim: Karla La Borrada, a 67-year-old trans singer and former sex worker, who Margolles befriended while working on an earlier photographic project. “But instead of doing one piece in her memory, I wanted to do something to represent the whole trans community – a collective piece about that community hugging her,” says the artist. Karla worked in the once glamorous border city of Ciudad Juárez, now one of Mexico’s most dangerous places. In 2023, it was second in Mexico’s murder league, behind only Tijuana, with 1,246 officially recorded killings. Yet in the prohibition era, the artist explains, Juárez was where Americans went to have themselves a ball. Frank Sinatra and Nat King Cole performed in its clubs. Marilyn Monroe travelled there in 1961 to file for divorce from Arthur Miller, and Steve McQueen died there in 1980. But by the time Margolles herself arrived, the celebrities had long gone and the clubs had disappeared, leaving only the outlines of rooms on crumbling walls and shiny fragments of dancefloor beneath the rubble. Her aim was to make a photographic record of these places, along with the women who once worked in them. Each woman was associated with a particular club and Karla, who specialised in ranchera music, helped her to track them down. “Karla was the pillar of the community. And on 22 December 2015, she was murdered,” says Margolles, flatly. I was an artist and observer developing this idea of the dead body as a reflection of society Her friend’s death was still fresh in her mind when she was invited to submit a proposal for the fourth plinth. In 2022, she put out a call for volunteers, imagining that she would delegate much of the work to her assistants, but she found herself unable to do so, “because trust is part of the project”. To win that trust, she works with two assistants, both trans: Mo Marin Mendez, from Mexico, and Sega, a London-based tattoo artist, who will themselves be memorialised in the work as two halves of a single cast. Their job, says Sega, was to hand-hold and tease out details of trans life from the volunteers that Margolles would not know about. Mexico is second only to Brazil in southern and Central America for homophobic and anti-trans violence, says Margolles. And Ciudad Juárez is a staging post for migrants from neighbouring countries desperate to get to the relative safety of the US; people like Oscar, who turned up while fleeing Venezuela, and who now sends regular updates of their new life. The artist speaks reverently of “the moment of silence, when [her volunteers] touch their face to put on the grease, and when they’re lying down with the plaster on their face. The migrants, who had histories of feeling silenced and unsafe in terrible conditions, they just lie down and stay there.” When the casts were removed, each was invited to make a playlist, as a first step towards talking about their lives. “Someone who is exhausted from walking, or feels unsafe because they have just been chased – they tell their story.” In London, she adds, “the experience was much calmer”. Margolles tracks her preoccupation with death back to her own childhood in the northern Mexican state of Sinaloa. It was, she says, “a very, very violent place. There’s not one family who don’t have friends or relatives who’ve been murdered.” While she is reluctant to talk about her own experience – “for me, the more invisible I am the better” – she concedes that this early proximity with murder meant that “unconsciously I empathised with grieving”. In her late teens, she followed an older sister to Mexico City, intending to study photography, but enrolled in the faculty of political science at Universidad Nacional Autonoma de Mexico to keep her family happy. The faculty was next door to the medical school, and she started hanging out with trainee doctors, attending their lectures and tagging along to the dissection rooms where they did their anatomy training. She went on to qualify as a forensic technician in order to gain legitimate access to morgues, she says. “Mainly I was an artist and observer developing this idea of the dead body as a reflection of society.” In 1990, she co-founded an artists’ collective, Proyecto Semefo (a contraction of the Mexican coroner’s office, or Servicio Médico Forense), which specialised in using the detritus of murder investigations to confront violence in the country. In 2009, she took an installation of blood-soaked blankets and sheets to the Venice Biennale, where it was nearly thrown out of the Mexican pavilion for drawing attention to the country’s war against drugs. Margolles is contemptuous of squeamishness about works that stare down the realities of death, decay and the horrifying results of violence or abuse on the tissues of the human body. “I’m afraid of both life and death, but you have to live with it,” she says. True horror is not what is left when life has run its course, but what happened along the way. “To look at the liver of a heavy drinker in an autopsy and see the damage they have inflicted on themselves – that is what should horrify us. Perhaps if people saw how delicate the body is, they would stop over-eating and drinking.” The artist’s support for trans people does not begin and end with the installation. She has worked with a shelter for LGBTQ+ migrants in Ciudad Juárez. Though all women are at risk from hate crimes, she says, trans women are particularly vulnerable because they tend to be very visible. In general, the trans community does all it can to make itself invisible in public places because everyone is afraid. “Now we are giving faces to them all.” She doesn’t yet know where the life stories or the photographs will take her, but the playlists will feature at the unveiling of the installation. As for the casts themselves, she is looking forward to them generating new life-forms as they moulder away under the London skies. Much like life itself, she says, “we know how it’s going to start. But we don’t know how it will finish.” Mil Veces un Instante will be on view in Trafalgar Square, London, from 18 September