Sea glass on granite and limestone: frosting, colours and what stays

Two coasts, two kinds of glass

The Maresme beaches are coarse granite sand, grains the size of lentils, and glass rolled in them comes out well rounded, evenly frosted and fairly small, rarely bigger than a fingernail. The Garraf coves are limestone, with pebble beaches and rock shelves, and the glass there tends to be larger, less evenly shaped, sometimes still with a sharp edge or a readable fragment of lettering, because it has been knocked around rather than ground down.

I find more glass in the Garraf coves, especially after a storm from the south-east, and better glass on the Maresme, where the sand does a slower and more thorough job. The difference was obvious within the first few months, when I started keeping finds from each coast in separate jars and the two jars looked like they came from different countries.

The best place to look is rarely the obvious one. On the Maresme it is the band of coarser grains at the very top of the wet sand on a falling sea, where the heavier pieces settle out, and on a calm day that band is only a stride wide. In the Garraf it is the back of the pebble beaches, where a winter storm has thrown stones up against the rock and left the glass tangled among them. Looking along the waterline, where most people walk, finds almost nothing on either coast.

Frosting and what it takes

The frosted surface of sea glass comes from two things working together: mechanical abrasion by sand and pebbles, and a slow chemical weathering of the glass surface by seawater, which leaches out some of its components and leaves a fine pitted layer. How long it takes depends so much on the beach that any figure is a guess, but a piece with a fully even frosting and no shine left is generally agreed to have been in the sea for decades, not years.

I leave anything that still has shine on it, a sharp edge, or a clear glassy patch. That glass is not finished, and there is no reason to take it off the beach early when the sea will do a better job with it in another twenty years. If it is sharp enough to cut a foot, I pick it up and put it in the litter bag rather than the glass jar, which feels like a reasonable compromise.

What the colours tell you

Most of what I find is white, pale green and brown, from ordinary bottles of the last half century. Paler aqua and a very light green come more often from older bottles and window glass, and I find them more often in the Garraf, near where old coastal buildings stood. Cobalt blue is rare and always exciting. Red, orange and yellow are so rare that in six years I have found two pieces of red and one I think is yellow, both on the same Maresme beach after the same storm.

I do not try to date pieces precisely, because I am not an expert and most of the dating guides assume American bottles. But a thick, slightly bubbly, pale green shard with a curve that suggests an old wine bottle is a different object from a thin clear square of window glass, and I note what I think each piece was in a small column beside the date and the beach.

Pottery comes up alongside the glass and follows the same rules. Blue-and-white fragments, pieces of glazed floor tile and the rim of what was once a heavy earthenware dish all turn up on the Garraf side, rounded by the sea in the same way. I keep a few of the best in the Garraf jar and put the rest back, and I note them in the log because the patterns tell me something about the houses that stood along that coast before the road was built.

The rules I keep about taking it home

I keep a jar per coast and I no longer let the jars grow. When one fills, I pick the pieces I like least and put them back on the same beach they came from, which is less sentimental than it sounds and makes the whole collection better. I never take glass from a protected area, from inside a rock pool or from somewhere it forms part of the structure, like the fragments cemented into old breakwaters, and I never take anything from a beach where someone is clearly collecting for a living.

The glass is the one exception to this notebook's leave-it-where-it-was rule, and it is an exception because it is rubbish that the sea has improved rather than a living thing or a piece of the shore. Even so, the jars are small on purpose. The pleasure is in the finding, the sorting on the kitchen table in the evening and the notes that go with each piece, not in owning a great deal of glass.