Turn up at a car boot sale and you will see them side by side: a stack of opal glass mixing bowls, some of them seventy years old, some made last year. From three feet away they look much the same. Up close they behave very differently, and so do their prices. Four clues do most of the work — the mark on the base, the weight in your hand, the way the colour sits on the glass, and how the oven-safe claim is written. Get those right and you will rarely be fooled.

Begin at the base

Every genuine piece of Pyrex carries a mark, and that mark is the first place to look. Vintage Corning Pyrex usually shows the name pressed into the glass rather than printed onto it, alongside a model number and often a capacity. Early twentieth-century pieces tend to be sparsely marked; mid-century ones frequently add a registered trademark note next to the name. British collectors have an extra detail to watch for: from the 1920s, Pyrex was made under licence by James A. Jobling in Sunderland, so "Made in England", "JAJ" or an English town name can appear on pieces that are perfectly period.

Modern glassware tends to be neater. Lettering is sharper, spacing is more even, and there is often a country of manufacture plus small symbols for dishwasher and microwave use. A base that looks printed by machine rather than pressed into thick glass is a decent hint that you are holding something recent.

Beware the completely unmarked piece, too. Opal glass was made by plenty of manufacturers, and a worn base can lose its mark entirely. No mark does not mean no age, but it does mean you cannot date a bowl on branding alone.

Weight, thickness and feel

Pick up a 1950s mixing bowl and a modern one of the same size and the difference is immediate. Vintage opal glass is dense, and the walls are thick. Because these pieces were pressed in moulds filled by hand, thickness often varies slightly around the circumference, and rims can be a touch irregular. Mould seams, where visible, tend to sit proud rather than being polished flush.

New glassware is more consistent: thinner walls, even rims, a smoother finish overall. It is not a foolproof test — heavy modern pieces exist and small vintage ones can be light — but combined with a base mark it is reliable enough.

Ignore the ring test

You will often see advice about flicking the rim and listening for a clear ring. It does not work. Pitch depends on the shape, the thickness and how you are holding the piece, not on its age. A dull thud tells you nothing except that you have just tapped a bowl in a shop.

Colour and decoration

Mid-century Pyrex decoration was fired on. Whether the colour was applied as a decal or as a painted band, the heat of the kiln bonded it into the surface, so a fingernail dragged across the edge of a pattern should meet very little resistance. The colour itself has a soft, slightly chalky density. It looks painted rather than photographed.

Look for honest wear as well. Genuine vintage pieces have lost colour along the rim, faded where bowls have been stacked, and picked up fine scratches across the busiest part of the design. A bowl that is bright, glossy and unmarked across sixty years of imagined use deserves a second look.

Modern prints are usually pad-printed or transferred, and can sit slightly proud, with glossy, high-contrast edges. The palette gives them away too. The classic range ran to turquoise, primrose yellow, pale pink, lime, dark green and orange-red; if a "vintage" piece is a saturated modern shade with a metallic finish, walk on.

Capitals, and what they mean for the oven

This is the one piece of Pyrex trivia worth memorising. The all-capitals PYREX mark is associated with borosilicate glass — the low-expansion glass that copes well with rapid temperature change. When the American consumer licence changed hands, the lowercase "pyrex" mark was used on soda-lime glass, which is tougher against knocks but far less forgiving if you move it straight from a cold fridge into a hot oven. In the UK and Europe, Pyrex sold by International Cookware is generally borosilicate, so capitals are a useful guide rather than a rule.

The safe approach with any piece whose history you do not know is to treat it gently. Never put glassware on a hob, under a grill or in direct contact with a hot ring. Do not move it between temperature extremes. Let a chilled dish come up to room temperature before it goes in the oven, and do not set a hot one down on a cold worktop. If the base carries a specific oven-safe statement, follow it. If you are unsure, use the dish for serving rather than roasting.

Glass rarely warns you before it fails. Sudden temperature change is the usual culprit, and it happens in seconds.

Shapes, patterns and numbers

Model numbers can help. The nesting mixing bowls most collectors know are the 400 series, and casserole dishes and refrigerator dishes each sit in their own numbering range. Odd numbers, missing lids and mismatched sizes are all normal in second-hand collections, since pieces were sold separately and rarely stayed together.

Pattern names are a pleasure — Gooseberry, Butterprint, Snowflake, Daisy, and British JAJ designs such as Chelsea — but they are also where people get caught out, because popular patterns have been reissued. A familiar design on thin, bright, sharply printed glass is a reproduction. Never buy on pattern alone.

A quick check before you commit

1. Turn it over. Look for a mark pressed into the glass, not printed on it.
2. Note the lettering case and any country of manufacture.
3. Weigh it against a similar modern bowl.
4. Run a fingernail across the edge of the decoration.
5. Check the rim and the busiest part of the pattern for wear.
6. Ask for a photograph of the base if you are buying online.

Buying and using it sensibly

No single clue is decisive. Marks wear away, weights overlap, and a good reproduction can be very close. Two or three clues pointing the same way is what you want before you pay a collector's price.

Check the rim for chips by running a fingernail around it; a chip will catch even when your eye misses it. Look for crazing — a fine web of cracks inside the glass — and if you find it, retire the piece to a shelf. Decorated surfaces that have worn or flaked are best kept away from food.

Finally, buy what you will actually use. Collectors' markets shift, and a pattern that was fashionable a few years ago can sit unsold today. A bowl you enjoy using every week is worth more to you than a speculative purchase that lives in a cupboard. And if you are ever uncertain whether an old dish is safe in the oven, do not test it with a Sunday roast — use it for salad and keep the mystery.

Photo: Pavel Danilyuk / Pexels