What is Black Sea Glass?

“Not all treasure is silver and gold, mate.”

~ Capt. Jack Sparrow

Olive green black glass
Typical Olive Green color of Black Glass
Black sea glass
One of the rarest "black glass" pieces in my collection
Black Glass Turns Out to be Red
Typical Black Glass Looks Amber or Olive Green

Where Does Black Glass Come From?

What is Black Glass? We get asked this question a lot. The term “Black Glass” is used because without a strong light source, the glass looks black. On a pebble beach, like Seaham Beach in England, it is almost impossible to tell the difference between black rocks and black sea glass. It is considered a rare color of sea glass because it can originate from bottles manufactured as long ago as the 1500s. It was primarily manufactured in Europe but was later adopted in the New World of America.

How Was Black Glass Made?

Black glass bottles were originally hand-blown as early as the late 1500s. The dark color primarily came from the addition of minerals such as iron oxide or coal ash.

In the beginning, they were formed into shaft-and-globes shapes starting in the mid 16th century, which then evolved into the “onion” shape of the 17th century, later evolving into the “mallet” shape.

Seaham was the location of the largest glass-bottle works in Britain. Founded by John Candlish in 1853, under the patronage of the 4th Marquess of Londonderry, the Londonderry Bottleworks turned out up to 20,000 hand-blown bottles a day. They used a variety of colors in it’s “Black Glass”, so you never know what color it will be when you find it. That glass dates from around 1850 – 1921 when the factory closed. 

Another kind of “Black Sea Glass” is called Vitrite. Vitrite is a kind of “slag glass” and was primarily used as a dielectric, or electrical insulator, generally found in the base of common incandescent light bulbs.

What Was Black Glass Used For?

The dark color protected the contents of the glass bottles from degradation from sunlight. It also contributed to the strength and heaviness of the glass. It was common to use black glass for bottles containing spirits, olive oil, vinegar, medicines, cider, or just about any of the liquid, up until the mid to late 1800s.

General Electric was one of the largest producers of incandescent bulbs along the shores of Lake Erie in Ohio. The more lumpy shapes are usually the glass slag that comes from the furnaces of the light bulb manufacturing facilities. Sea glass hunters still do find some discarded shards in that area. We also found glass insulator pieces on the shores of the Mediterranean in Spain (see picture of red glass below). There are also brighter colors of insulator glass in yellow and blue.

The term “slag glass” also refers to a type of decorative glass primarily used in the 19th century to create beautiful marbled pieces designed to resemble malachite or other striped stones.

Black Glass Found in the Mediterranean

Being considered quite rare, it is usually only found in specific areas, not just randomly on beaches with other sea glass. 

The black glass that we found on the Mediterranean Coast of Spain was old, more like 200-400 years old because is was typically used on old sailing ships. 

Black sea glass is even more rare when you find a color other than the typical amber/olive. The piece pictured below is from an insulator used in light bulbs. This piece is the only red one that I’ve ever found. These are easier to spot because of their unusual shape. Insulator pieces are also found in dark blues and purples. Most of the typical olive green black glass is older and more rounded, looking like a pebble.

"Black Glass" used as Insulators for Light Bulbs
Red insulator glass
The Same Piece Held Over a Light