Monday, December 18, 2017

Like Snow From Heaven

28th December 2005 in Florence
Photo by Marco De La Pierre
In its simplest incarnation, glass is nothing more than crushed quartz or sand mixed with 'glass salt'. This salt is the alkali carbonates of sodium or potassium. Essentially, the ash of certain plants that reduces to oxide in the furnace and dramatically lowers the melting point of the sand.

In Pliny's ancient account of the discovery of glass, the process takes place in a single step. The ingredients came together accidentally in a fire and "glass trickled out." The problem is that pure quartz does not react with the salt until it gets very hot and even then, it does so reluctantly. In a wood-fired furnace, quartz stones, even small pebbles or sand would melt with excruciating slowness. As Neri advises, "... this would only succeed after a protracted period of time and a great amount of trouble." 

'Fritting' is an intermediate step that speeds the process considerably. Glassmakers reduce the quartz to a fine powder and then mix it with the alkali salt. In the heat of a kiln, the entire content of each stone is thus exposed to the salt right from the beginning. This roasting process starts a chemical reaction between the ingredients. In the late Renaissance, the combination was then cooled and 'aged' for several months before use. When made from pure quartz river stones and the best Levantine ash, the result was what Neri calls 'bollito,' "white and pure like snow from heaven."

A third ingredient of glass, critical to its long-term stability is lime, or calcium oxide. Without the lime, glass is susceptible to attack by water. The water actually dissolves the glass, or less dramatically makes it subject to 'glass disease' or 'crizzling', a condition where the glass slowly decomposes due to humidity in the air. Waterglass is a product made without the lime. On Murano and elsewhere, it was dissolved in water and painted onto the shells of eggs to seal and preserve them. In Neri's era, lime was produced by roasting seashells. It was a key ingredient of cement, and as such was readily available. It had been a major commodity throughout the Mediterranean since the Roman Empire. Neri advises to add lime to all his glass recipes, but it is not so clear that he himself understood why it was so important.

*This post first appeared here on 11 October 2013.


  1. interesting as always.
    Do you have any idea where i can look for these plants?

    1. The species that Neri used grow in sandy costal areas all around the Mediterranean. Salsola kali is sometimes sautéed with olive oil as a veggie. In Italy this variety is known as “barba di frate” or “friar’s beard” in English :-). Closely related plants (such as tumbleweed) grow around the world, but most lack the high sodium carbonate content good for glassmaking. Thanks for reading!