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How sea glass gets its different colours

Sea Glass Colors

Sea glass comes in an amazingly wide range of colours and hues, but have you ever wondered how that glass got its colour, well here’s a little look at the science of colouring glass.

90% of glass is made from a standard mix of Silicon Dioxide (70%); Calcium Oxide (14%); Sodium Oxide (16%) percentages may vary slightly, with various trace elements added to provide the colour.

A number of materials, mostly metal compounds, are commonly used to colour glass, including cobalt, lead, uranium, copper and even gold.

A glass container’s colour was chosen to make the product more appealing, the same perfume or drink can sell for a premium in a sapphire blue bottle compared to a clear one.

These days, glass makers can precisely control the colour they wish to achieve, but centuries ago, it was coloured through random experiments, with varying degrees of success.

Over a millennium ago. Abu Musa Jabir ibn Hayyan, a Persian chemist, concocted dozens of recipes for colouring glass. He discovered that metal oxidation was the catalyst for colouring, earning him the title ‘The Father of Chemistry’.

Back in the day

After this discovery, colouring glass soon became a major industry. Firstly it was small bottles and ornaments, but soon, religious groups demanded custom coloured glass, and stained glass was born. However, there was a problem: Those gorgeous colours didn’t last for very long. Red in particular faded quickly. After a little more experimenting, craftsmen found that adding a little gold to red preserved the color, a method that’s still used today.

Modern glass colouring

Today, manufacturers have precise, tried and tested recipes. You still have to add metal for the brightest, longest-lasting colours. Sulfide and powdered oxides are the most common. To create yellow, you add cadmium sulfide; red still requires gold chloride; a blue/violet hue needs cobalt oxide; purple needs manganese dioxide; pure violet requires nickel oxide; and emerald green calls for chromic oxide.

These are just a few of the options available. Additional colours can include fluorescent yellows and greens, ambers, whites and browns.

How glass gets its colour

Some of the main colours

AQUA, AQUAMARINE glass, also known as SEAFOAM is a “natural” result of the iron impurities found in most sands (Silicon Dioxide). It is very rare, if not unknown, for sand not to contain some traces of iron.  Sand deposits with very low iron content are a highly valued commodity. 

Aqua glass is the result of sand which had a relatively low amount of iron, and therefore didn’t require the addition of decolourizing agents (see ‘Clear Glass’ below).

Higher levels of iron produce darker greens, black glass, and even amber. Natural aqua glass was often called “green glass,” “bottle glass,” or “bottle glass green” by glass makers

COLOURLESS glass is rarely absolutely colourless. It will usually have very faint tints of pink, amber, grayish green, gray, or grayish blue. These faint colours can be seen easiest when looking through a thick piece of glass, i.e., side on. Colourless glass is usually produced by using the purest sand source possible and by adding “decolourizing agents” to offset the residual iron impurities. Commonly used for this were manganese dioxide, selenium dioxide (usually together with cobalt oxide), antimony and arsenic oxide – which is also used as a stabilizer of selenium in decolourizing glass – or some combination of these compounds

One result of the use of Manganese Dioxide is that glass decolourised using this compound will, upon exposure to sunlight, turn a light pink or lavender to a moderately dark amethyst or even a deep purple depending on the amount of manganese in the glass mix and amount of ultraviolet that it is exposed to..

OPAQUE WHITE glass – commonly called milk glass but sometimes called opal or white glass – was produced by the addition of tin or zinc oxide, fluorides (fluorspar), and phosphates.  It was also created by adding calcium and phosphate rich animal horns, bones, and even “bat guano” to the glass batch.

GREEN glass. There are probably more different shades of green to be found than any other colour.  It rivals the multitude of amber glass variations which can grade into various greens. The different greens were formed by a myriad of different colouring agents, impurities, and/or glass making processes. Iron, chromium, and copper all produce different green glass. Chromium oxide will produce yellowish green under oxidizing conditions and emerald green under certain conditions in the glass furnace. Combinations such as cobalt mixed with chromium will produce blue-green/teal glass 

AMBER glass is very common and like the greens above, amber colours were a result of the natural impurities in glass (i.e., iron & manganese) as well as from colour additives such as nickel, sulfur, black lead and in particular carbon, which was added to the glass batch in the form of coal, charcoal, or even wood chips. 

BLUE glass, from moderate to intense, is usually produced with the addition of the colouring agent cobalt oxide to the glass. Copper can also produce types of blue glass depending on the batch ingredients and temperatures used.

PURPLE, AMETHYST and RED were usually a result of glass that was coloured with nickel or manganese oxides and sometimes selenium, with true red usually a result of the use of an oxide of gold.  As noted in the colorless glass description above, small amounts of manganese dioxide was used as a decolourizer to offset the iron impurities present in virtually all sands. This colourless glass will variably turn amethyst upon long term exposure to sunlight.  With larger concentrations of these substances in the glass batch amethyst to purple glass is purposefully created

BLACK glass is a form of green glass which is so deep a colour as to appear black in reflected light and even in direct light  This strong and resilient glass was also a colour that offered the most protection to the contents from the effects of direct light. Most black glass is actually a very dark olive green or olive amber.  These types of black glass were the result of the same impurities or colouring agents as the olive colors, usually high iron concentrations but also other substances including carbon (from various sources including ashes and coal clinkers), copper with iron, and magnesia.

Well I hope you found this article informative. I will be looking in more detail at individual colours in the near future.


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What is Slag Glass and where does it come from?

Sea tumbled Slag Glass

Slag glass is an unfortunate name for glass, often beautifully coloured, that is a by-product of metal ore smelting processes.

Metal furnace slag can range from glassy to dull and stony in appearance and, when scratched, can range from hard to chalky. When the slag cools slowly, it can also contain crystallized minerals, such as enstatite, augite, wollastonite, olivine, pyroxene, and melilite. Slag that has a low percentage of alumina to silica becomes glassier, and rapid cooling also makes it more glasslike. 

Slag was often used as a building material, and as railway ballast, among other things, and according to one source, also for sea walls which would explain its presence on beaches.

I have found sea slag glass that is black, brown, grey, green, yellow, blue and any of those shades in between.

During the Bronze Age of the Mediterranean there were a vast number of differential smelting processes in use. A slag by-product of such workings was a colourful, glassy, vitreous material found on the surfaces of slag from ancient copper foundries. It was primarily blue or green and was formerly chipped away and melted down to make glassware products and jewelry. It was also ground into powder to add to glazes for use in ceramics.

Historically, the re-smelting of iron ore slag was common practice, as improved smelting techniques permitted greater iron yields – in some cases exceeding that which was originally achieved. During the early 20th century, iron ore slag was also ground to a powder and used to make agate glass, also known as slag glass.

Slag Glass
Slag glass as it would have looked before ending up in the sea.
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Safety glass and its hidden wonders

Safety glass is a term given to a range of glass types which are toughened through tempering, laminating or reinforcing with various materials.

In the world of sea glass it refers to glass that has originated from sheet glass that is reinforced with wire.

First invented and patented in 1892 by American entrepreneur and prolific inventor Frank Shuman, wire mesh glass, also known as Georgian Wired Glass, is glass which has a grid or mesh of thin metal wire embedded in it.

Beatifully frosted

Although it is often believed that the wire reinforcement strengthened the glass, it in fact weakened it, but its value was in its fire-resistant qualities and held together when broken rather than creating dangerous shards. For this reason it was widely used in institutional buildings such as schools and hospitals.

In recent years new materials and techniques have meant that wired safety glass has fallen out of favour and has been replaced.

Wired safety glass is mostly clear which makes for a white sea glass, and the wire mesh can have various patterns. I have picked some wired sea glass in other colours such as teal, brown or yellow, but not in any quantity.

The most attractive safety glass is well rounded, pleasantly shaped, and rust free. Here in the south of Spain, the hot dry winds immediately dry the glass once it is exposed on the beach reducing the opportunity for the wire to oxidise. If you live in damper climes be careful not to store damp wired sea glass with other glass as it can lead to rust stains which are virtually impossible to remove.

Safety glass in various colours found by Anna Varone

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A sunny day, but a grey day

It was one of those times when, after 5 hours on the beach, you think it’s time to go and grab a bite to eat, but the other half says “just another 15 minutes”, and aren’t you glad she did when within a couple of those minutes you stumble upon this beautiful dark grey.

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Black Sea Glass or Pirate Glass

Black Sea Glass

Black glass, is not actually black but generally olive green, amber or sometimes blue, and gets its colour from the addition of iron oxide which was incorporated to protect the contents of bottles from light damage.

The use of black glass was common up until the end of Nineteenth Century, so any pieces of black sea glass will be over 100 years old.

Black glass is difficult to spot because the frosting from many years in the sea makes it look like a stone and takes a practiced eye to spot on the beach, which makes it quite rare to find in collections.

There has in recent years been a fashion to call black glass, “Pirate Glass”. This is more of a marketing ploy than being representative of the glass’s origins.

Although most of the black glass we pick up in the South of Spain has less romantic origins, the southern coastline of Spain was subject to extensive raids by Barbary Corsairs or Pirates, operating from the Barbary Coast of North Africa (Morocco, Algeria, and Tunisia) from the 16th to 19th centuries*. So there is a chance that some of this black glass is actually “Pirate Glass” after all!

*Ref: Barbary Pirates