There is a particular pleasure in turning over a dusty bottle at a Saturday market and wondering who filled it, when, and with what. Glass is wonderfully stubborn stuff, and it carries the fingerprints of its own manufacture — seams, scars, colour shifts and lip finishes that can narrow a date down to a decade or two. You do not need a laboratory, just decent light, a magnifier and the habit of looking in the right places.

Read the Seams First

Tilt the bottle towards a window or a lamp. A faint raised line running up the body is a mould seam, and on most bottles it is the single most useful dating clue you will find.

• Three-piece moulds (roughly pre-1880s). The seam runs up the body and fades out somewhere around the shoulder, because the neck and lip were formed separately by hand. On many early bottles the seam disappears altogether below the shoulder.
• Two-piece moulds with a hand-finished lip (about 1880 to 1910). The seam climbs the neck but stops short of the lip, which was still tooled or reheated over a flame to smooth it.
• Machine-made bottles (from the first decade of the twentieth century). The seam runs unbroken through the lip to the very top, because body, shoulder, neck and lip all came out of one mould in one go.

Run a fingernail along the line rather than trusting your eyes; a seam polished away by a glassworks finishing shop can be almost invisible. Equally, do not read too much into a single seam. Look for ripples or faint whittle marks inside the neck too — these suggest a wooden mould, which points to the earlier nineteenth century.

Pontil Marks and the Base

Turn the bottle over. The base is where the glassmaker's tools left their most honest signatures.

• Open pontil. A rough, sharp, irregular scar, sometimes gritty or slightly rusty in colour, left by the iron rod that held the bottle while the lip was finished. Generally pre-1860s.
• Improved or snap-case pontil. A neater ring or shallow depression, with a smooth edge rather than a jagged one. Mid-nineteenth century.
• Ground or polished pontil. The scar has been smoothed flat on a stone wheel, often found on better decanters and finer ware. Early to mid-nineteenth century.
• Valve or suction scar. A circular dimple with a ring around it, sometimes off-centre and slightly cloudy. That is a machine mark, so the bottle is twentieth century.

A completely plain base does not automatically mean modern. From roughly the 1850s onwards, moulded bottles were often rested on their own base inside the mould, leaving no pontil at all. Look instead for honest wear: fine scratches, small nibbles and a dulled ring where the bottle stood for a century or more.

What the Colour Is Telling You

Glass colour comes from the sand, the impurities in it and the chemicals added to clarify it, and each era had its favourites.

• Pale aqua and sea green. Iron in the sand, and the default colour of everyday bottles for well over a century.
• Deep olive, near-black glass. Hold it to the light and it glows dark green or amber. Typical of eighteenth and early nineteenth-century wine, cider and spirits bottles.
• Cobalt blue. Costly to make and usually reserved for apothecary, poison and fancy goods.
• Clear glass with a lilac or amethyst tinge. Manganese was used as a decolouriser from about the 1880s, and it turns purple in sunlight. A clear bottle that has gone pale violet was almost certainly made between roughly 1880 and 1915.
• Clear glass with a faint straw or pinkish blush. Selenium replaced manganese around the First World War, giving a much subtler sun-changed colour.

Never date a bottle on colour alone. Staining from contents, burial in different soils and long exposure to sun all shift the appearance of glass, and two bottles from the same mould can look quite different after a century apart.

Lips, Finishes and Closures

The mouth of a bottle is another reliable clock. A cracked-off and fire-polished lip — simply snapped from the blowpipe and rounded in the heat — is usually early. An applied lip, where a separate ring of glass was wrapped around the neck, shows a fine seam underneath and points to the nineteenth century. A tooled lip, neatly shaped with a pair of tongs, belongs mostly to the late Victorian and Edwardian years.

Closures narrow things further. Internal screw threads, cut inside the neck for a stopper, appear from the 1860s onwards. External screw threads — the sort a modern jam jar has — are largely a twentieth-century development, becoming common from the 1920s. Crown caps arrived in the 1890s, while swing-top wire closures date from the 1870s. Rubber-ring vacuum seals belong to the 1880s and later.

Shape, Embossing and Registered Marks

Shape follows function, and function follows fashion. Torpedo and ballast bottles, designed to lie on their sides so the cork stayed wet, gave way to the familiar upright mineral water bottle, then to the marble-stoppered types popular from the 1870s. Fluted or ribbed poison bottles, often embossed with a warning, date from the 1870s onwards, when legislation encouraged them.

Embossing was cut into the mould by hand, so the lettering on genuinely old bottles is often slightly uneven, shallow or crowded. Look for a small diamond-shaped registration mark, used in Britain between 1842 and 1883 — it can pin a design down to a precise year once you learn to read it. Export wording such as Made in England generally indicates the late Victorian period or later.

Telling Genuine Wear from Staged Age

Reproduction bottles are common, and most give themselves away fairly quickly. A crisp seam running over the lip on a bottle described as Georgian is a warning sign, as is glass that is uniformly coloured with no variation at all from neck to base. Modern copies often sit perfectly flat and unmarked underneath, whereas a genuinely old bottle will have an uneven base, a worn ring of contact and a scatter of tiny scratches from decades of sliding across shelves.

Be wary of painted-on grime, acid-etched "patina" and stoppers that clearly belong to another bottle. Check inside the neck and under the shoulder for dirt that has settled naturally into crevices, and handle the piece in raking light — fakers rarely bother with the parts you cannot see. Once you get a bottle home, wash it gently in warm water with a drop of washing-up liquid, never in a dishwasher, and keep coloured glass out of strong sunlight so the colour you have just learned to read does not fade away.