"Quieres comer?" asks Aldo, making the universal gesture for eating as he slows the car and pulls over by a narrow arched opening in an old stone wall. A long-term chauffeur for tourists in this part of Peru, Aldo knows the region well: he was born in Puno on the banks of Lake Titicaca, some 14 miles southeast of our present location, and speaks both local languages of Quechua and Aymara.
Sitting near the roadside, backs against the cold stones of the thick wall, are two Quechua ladies of uncertain age and a man whose weathered face is just about visible in the heavy shade beneath his wide-brimmed hat. A llama and a couple of alpacas graze quietly by the road.
Aldo exchanges some brief greetings in Quechua and I say "goodday" in Spanish, to which I receive a couple of quiet responses and a tip of the hat from the man of the house. Behind the stone structure, a bare road leads through a windswept expanse of stunted scrub to the pre-Inca burial towers of Sillustani.
Aldo, who seems far hungrier than I, asks the women a question in Quechua. She responds with a quick come-hither hand gesture and walks through an arched doorway into a small stone-walled courtyard beyond.
"Come, we'll eat," says Aldo. "She always has food for the morning tourists."
And there, in a simple clay bowl on a stone table, is an assortment of Peruvian tubers: boiled yellow potatoes, golden ulluco, and white and black chuño potatoes. The woman sticks a knife in a round of snow-white cow's cheese.
"Queso," she says, before turning again to tend her smouldering fire.
"What's the sauce?" I ask Aldo, pointing to a bowl containing a khaki-coloured, slurry-like liquid.
"Chaco. Arcilla de chaco," he replies: "Clay."
Officially called geophagy, Merriam-Webster defines "the practice of eating earthy substances (as clay)" as a practice performed by humans to "augment a scanty or mineral-deficient diet or as part of a cultural tradition." Animals do it too: parrots, chimpanzees, and sulphur-crested cockatoos.
Not that I know any of this as I eye the yellow-brown sauce offered to me. The woman comes and mixes it with a large wooden spoon.
"Chaco," she says, half smiling as she taps her stomach with her fingertips. She passes me a clay bowl of warm potatoes. I take one, dip it in the clay and take a big bite.
You know how you imagine clay would taste? Chaco sauce tastes just like that. With added salt.
As we eat our clay and potatoes, Aldo tells me that the inhabitants of the Altiplano area of Peru and Bolivia have been eating clay, known as arcilla since pre-Columbian times, more for its medicinal properties than for its earthy taste. For peasant farmers—and in particular, the pregnant wives of peasant farmers—clay is an important source of calcium, iron, zinc, and copper.
Today, Aldo explains, these rural farmers eat clay primarily to ward off or treat indigestion and stomach ulcers (similar to the commercial medication Kaopectate, whose original active ingredient was a type of absorbent clay). Clay is also thought to have detoxifying properties, driving Hollywood's recent clay-eating fad.
But the good people of the Altiplano don't just dig up any old clay for lunch. If they did, the potential health benefits would be seriously offset by the high chance of ingesting bacteria, viruses, and parasites that live in the substance, or of poisoning themselves through the intake of high levels of lead or arsenic.
"Over there," says Aldo, gesturing over the stone wall and toward the rugged landscape dotted with clumps of hardy Peruvian feather grass, "is where the local farmers dig the holes to get the chaco. They dig holes deeper than themselves so they can get down to the pure clay. Clay mixed with other minerals—dirty clay—is no good."
Aldo pulls a greyish shard of dry chaco clay from among a pile of fire-blackened pots. Passing it to me, he explains how the clay is dug out of the pit with a pick. But chaco, he says, is not eaten straight from the ground: it should first be fully dried in the sun. Only once it has dried is it ready to be dissolved in water with salt, after which it is ready to eat with potatoes and other highland tubers.
"Try this" says Aldo, handing me a wrinkled, freeze-dried chuño potato. It is bitter and not entirely pleasant, but the lady gives me a knowing smile.
"Now dip it in the clay," he instructs.
Coated in the salted chaco, the bitterness of the chuño is almost completely counteracted. Clay is not just medicinal, then, but also an ingredient for making undesirable tastes more tolerable.
And the story of chaco doesn't end here, on this sparsely-populated high plateau. Head some 540 miles northwest of Puno and you'll touch down in Lima, Peru's vibrant coastal capital. Here, among an ever-increasing number of globally acclaimed restaurants, is Central, voted the fourth best restaurant in the world in 2016.
Chef Virgílio Martínez and his team traveled all over Peru, from the desert to the highlands to the depths of the jungle, to find new ingredients from traditional sources. One such ingredient is arcilla de chaco, sourced from Acora in the Puno Region.
"It was a whole new discovery for us about ten years ago when we four chefs were on our way to stay a night on Lake Titicaca," Martínez tells me when I ask about the unusual dish. "We were looking for food, traditions, new experiences—the whole package—but honestly, we were more motivated to see the freeze-dried potato. After a long walk on the Altiplano, we saw a group of farmers just scratching clay from the soil. We asked them what it was for and they said it was to bake their recent harvest of potatoes. About 30 varieties of potatoes were covered with the clay and then baked in a very rustic, amazing oven."
Martínez describes the raw clay as having a gentle and sweet herbal flavour, with a soft texture not unlike gum. But by the time the clay has reached Lima from Puno, it's mostly dry and is often turned into a powder. At this stage, says Martínez, there are no rules as to how it is used in the restaurant.
At Central, chaco clay forms part of the "Green Highlands" plate with cushuro (a freshwater alga sometimes known as "Andean caviar") and cacao. The clay is also used in a number of other innovative ways.
"We still cover potatoes with it but we also use it to thicken sauces and preparations," Martínez explains. "For sweets and sweet crusts, for emulsions like cold sauces, and to cover fish before they are baked. We melt clay with cacao for desserts. And we cover vegetables in clay before they are cooked, after which you have an edible crust."
Thanks to such culinary innovations, chaco clay seems to have found a new home beyond the stone walls of the Altiplano peasant farmers, and away from the flash-in-the-pan fads of Hollywood's ever-detoxifying celebrities.