Can You Pour Boiling Water Into a Glass Jar?

No. Kilner's own product pages say plainly: "Do not pour boiling water directly into your jars/bottles"¹. That instruction appears on the preserve jars, the clip top jars, the twist top jars, the push top storage jars and the milk bottle, so it is not a quirk of one size or one lid type. The maker's wording is the same across the range, so treat it as the rule for every Kilner jar and bottle you own¹.

The reason is the glass itself. A sudden change in temperature puts stress into glass, and a jar that is cold when boiling water hits it can crack or shatter, sending hot water and broken glass across the kitchen. The fire and rescue services make the same point about glass containers generally: "Glass containers can overheat and may crack or shatter"². A cracked jar is a nuisance; a shattered one full of boiling water is a scald.

What you can do instead is simple. Warm the jar gradually first, fill it with liquid that has cooled a little from a rolling boil, and stand it on a dry cloth or wooden board rather than a cold worktop. If you are preserving, follow the maker's leaflet that comes inside the jar¹. For the wider job of getting jars clean and ready, see How to Sterilise Jars for Preserving.

What the warning covers: preserve jars and storage jars

The instruction is repeated across the range, which tells you it is a property of the glass rather than of one product. The preserve jar, described as perfect for storing and preserving healthy, home grown foods and for pickling vegetables, preserving whole fruits and chunky chutneys, carries the warning and also contains a detailed instruction leaflet inside¹. The set of three preserve jars, with a classic clip-top design and durable glass construction, carries it too, and is listed as dishwasher safe³. The wide mouth preserve jar and the tomato fruit preserve jar carry the same line⁹.

The clip top jars follow the same pattern. The square clip top jars and the set of three clip top square jars all carry the warning¹¹. So do the facetted clip top jar and its set of three¹⁶. The twist top jars, from the hexagonal size up to the round jar, carry it as well¹⁸. The round twist top jar, sold for preserving batches of home-made jams, jellies, marmalades and chutneys, is no exception¹².

The push top storage jars deserve a word of their own because they look like the obvious vessel for a hot drink or a batch of stock. They are not. The universal jars are described as suitable for a variety of storage solutions in the kitchen, from countertop biscuits, sweets and treats to dried food ingredients in cupboards and pantries⁴. The smallest of them is described the same way, and its push top glass lid incorporates a high quality silicone gasket to provide a secure and airtight seal keeping dry food fresh²⁰. The largest jar is handwash only and should be washed before first use⁸. The other two also say to wash before first use⁴. None of that is a hot liquid job.

Where boiling water must never go: straight into the jar, and never onto a pan fire

The jar warning is one example of a wider rule that the fire and rescue services repeat constantly: boiling water and hot fat do not mix. Never add water to hot oil or a pan that has caught fire, because it causes a reaction which could lead to burn injuries²¹. If you have a cooking fire, never throw water over it or use a fire extinguisher, as the effects can be devastating and spread the fire further⁶. Never throw water on a burning pan: get out, stay out and call 999⁷. One fire service puts it bluntly: "NEVER throw water over the pan"²². Another simply says: "Never throw water over it"²³.

The same caution applies to any glass container near a heat source. The fire and rescue services warn that glass containers can overheat and may crack or shatter, which is why a jar candle should never be left to burn down to the glass². A jar that has held a candle is not a jar for boiling water.

There is a second, quieter reason to keep boiling water out of jars: it is easy to scald yourself lifting a full jar. Fill jars on a stable surface, keep your hands and face away from the opening, and let the jar stand before you move it. If you are filling jars for a store cupboard, the Emergency Food section covers what keeps and what does not.

How to use Kilner jars without cracking them

The maker's warning is about a direct pour, not about jars in general. Kilner jars are built for preserving and storage, and the range is wide enough to match the job. Preserve jars are for pickling vegetables, preserving whole fruits and chunky chutneys¹. Clip top jars are described as perfect for preserving healthy, home grown foods¹¹. Twist top jars suit jams, jellies, marmalades and chutneys¹². Push top storage jars are for dry goods, with a silicone gasket that keeps dry food fresh²⁰.

A practical routine that respects the warning:

1. Wash the jar before first use, as the maker instructs for the push top jars⁴.
2. Warm the jar gradually with warm, not boiling, water.
3. Let boiled liquid stand briefly before filling, and pour through a jug or funnel.
4. Stand the filled jar on a cloth or board, not a cold worktop.
5. Read the instruction leaflet that comes inside the preserve jar¹.

For the full method, including oven and dishwasher routes, see How to Sterilise Jars for Preserving. If you are deciding between glass and other storage, Mylar Bags or Glass Jars for Dry Food sets the trade-offs out side by side, and the Kilner Clip Top Jar page covers that style in detail.

Kilner jars: an English design over 180 years old

The Kilner jar is not a recent import. The maker states that "The original Kilner® Jar, invented by John Kilner and Co. in England is as much loved and successful now as it was over 180 years ago"⁸. The same sentence appears across the product pages, from the round twist top jar to the set of three clip top square jars¹⁸. So the answer to where Kilner jars were invented is England, and the answer to how old the design is, over 180 years.

That history matters to a household because it explains the range. The maker's pages cover preserve jars, clip top jars, twist top jars, push top storage jars and a milk bottle, and the same boiling water warning appears on each of them¹. The clip top jar is the design most people picture, with its glass lid, rubber seal and metal clip, and it sits alongside the screw lid and storage jar versions in the same catalogue¹.

Boiling water in a power cut or a boil water notice

Households often reach for a jar when the kettle is the only hot water they have. Two situations are worth separating.

If your water supplier has issued a boil water notice, the advice is about drinking water, not about jars. You should never drink water from the hot tap; the kitchen cold tap is usually the only tap directly connected to the mains supply and should normally be used for drinking and cooking²⁴. Boil the water, let it cool, and store it in a clean container. If your water has bits in it, pour the clean water from the top of the jug for drinking or cooking with²⁵.

If the power is off, the advice from National Grid and from local resilience guidance is to boil some water and keep it in a thermos flask, so you can use it to make hot drinks or fill a hot water bottle if it gets cold²⁶. A thermos flask, not a glass jar, is the right vessel: it is built to hold heat and it will not shatter when you pour. Hot water bottles do expire, and old bottles can break and cause serious burns, so check yours before filling it²⁸.

If your tap water tastes or smells odd, the cause may be the kettle rather than the water. The Drinking Water Inspectorate suggests confirming this by boiling some water in a saucepan instead; if there is no smell or taste, the kettle is the likely cause²⁹. Scotland's drinking water regulator gives the same test and adds that purchasing a different kettle without plastic in contact with the water should resolve it³⁰.