What did they use the very large ones for?

Big display jars sat in grocers and sweet shops. Shopkeepers bought loose goods in bulk and needed a clean, dry way to show them off to customers. Chemists also used substantial carboys in their windows. They filled them with coloured water rather than stock, drawing the eye of people passing in the street.

My grandmother left me a collection of old preserving jars with metal clips and rubber rings, but I have no idea how they were actually sealed when she used them for jam making?

The rubber ring sat on the glass rim. A glass lid went on top, and the metal bail wire pulled everything down tight. You heated the filled jar in a water bath. The heat forced air out, and as it cooled down, a vacuum formed inside. That vacuum kept the lid held down tight, sealing the food away from airborne bacteria until winter.

Why do some have purple glass?

Glassmakers often added manganese to the batch. They used it to counteract the natural green tint caused by iron impurities in the sand, aiming for clear glass. When exposed to the sun for long periods, the manganese reacts. It slowly turns the glass a light purple or amethyst shade over decades.

I keep finding jars with numbers embossed on the base, and some of my friends think they are dates or patent numbers, but what do they actually mean?

Those numbers are usually mould marks. When a factory ran multiple iron moulds on a production line, they numbered each one. If a specific jar came off the line with a flaw, the foreman checked the base. The number told him exactly which mould needed repairing or replacing, saving him from checking the whole line.

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Antique Glass Jars Dated 1900 - 1920

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Antique glass jars are sturdy utilitarian vessels blown or moulded from basic glass recipes, designed to store everything from apothecary powders and preserved fruit to boiled sweets.