Tuesday, July 16, 2013

Limestone in Mesoamerica – one of Mexico’s most versatile resources in still widely used today

On a trip to Todos Santos Cucumatan in Guatemala's highlands I witnessed a process that goes on all the time here in Mexico, but out of sight. I had stopped at a family-operated limestone quarry. I could see where blocks of limestone had been cut out of a thick deposit. But even more facinating were the fire pits where limestone was being subjected to intense heat in order to reduce it to powder.  I came home with a football-sized piece of fired-limestone which, for lack of a better place, I put on top of my filing cabinet.  I've watched it disintegrate into powder over the span of a couple of years.  It reminds me just how fundamental limestone has been in Mesoamerican life.

Limestone can be found throughout Mesoamerica. Mexico's mountain ranges and ridges are full of limestone. The Yucatan Peninsula is a limestone shelf. You'll know you're in an area where limestone is plentiful wherever you see a cement factory.  In fact limestone is the principal ingredient in Portland Cement, the most common type of cement used in modern construction.  In addition to being used as mortar, limestone is the basis of concrete, stucco, and grout.

It's a very soft stone when in the ground and easy to cut and shape with stone and wooden tools, which is all the ancient Mesoamericans had.  In fact it is so soft that ground water cuts through it easily, often producing interconnected caves. You can visit an extensive limestone cave system at the Grutas de Cacahuamilpa, a three-hour drive from Mexico City in the state of Guerrero. Similarly the Yucatan Peninsula is honeycombed with cave systems. Snorkelers love to explore them.

When limestone is put in direct sunlight it dries out and becomes a hard and long-lasting building material.  The longer it is out in the sun the harder it becomes. 

Architects in ancient Mesoamerica would place orders with the nearby quarry for limestone building blocks. The blocks of stone were usually the size and weight that one person could carry. Architects would request blocks that were smooth on one side for walls or floors, or smooth on two sides for steps or corners. Irregularly shaped blocks made up the solid fill of buildings.

Small pieces of limestone where heated in the kilns. The resulting fine white powder was mixed with water to become stucco. Add sand and gravel to the mix and it became similar to cement or concrete.

In areas of Mesoamerica where limestone is abundant whole buildings and pyramids were built of limestone building blocks held together with limestone cement. You’d never have seen the building blocks though. The outside surfaces were always covered with limestone stucco. They’d put powder from ground up minerals of different colors into the last layer of stucco to produce brightly colored walls. Unfortunately stucco is the least durable use of limestone and has worn off most ancient buildings.

A serious downside to dependence on limestone mortar and stucco is that it is greedy for firewood. Twelve times more wood than stone is needed to heat the fires to reduce the limestone gravel to powder. Many archeologists trace the demise of Teotihucan -- the world's largest city in 550 A.D. -- to the denuding of the forests around the city. With no forested cover on the mountains, the summer rainfall rushed down the mountainsides, untrapped by the natural aquifer inside the mountains. This in turn led to a lack of water for the city's population that may have reached 250,000. 

Powdered limestone was not just used in construction. It has also been a key ingredient in the Mesoamerican diet in the form of tortillas. Corn was the staple grain of ancient Mesoamerica's diet and continues to be so.  Unless it is first nixtamalized, corn eaten in quantity will lead to debilitating diseases caused by a niacin (vitamin B3) deficiency.  Nixtalamization involves soaking and cooking kernels of corn in water and powdered limestone. This process breaks down the hull and transforms the nutrients in the kernel making them accessible to the human body.  When eaten along with beans and chile our bodies can transform the mix into protein.

Ground nixtamal becomes the dough or "masa" from which a tortilla is made.  It's only ingredients are corn, limestone powder, and water.  It is patted into the shape of a tortilla and cooked on a comal.  An additional health benefit of limestone in tortillas is the calcium it adds to the Mesoamerican diet.     

I find it elegant how a plentiful natural resource such as limestone has been and remains an integral part of Mesoamerican life.  And not just as a construction material but also as an essential part of the traditional Mesoamerican diet. 

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