Avoid Thermal Shock – Prepare Your Chiminea Properly

You’ve bought a fabulous new clay chiminea from us. It arrives in perfect nick. You assemble it in your garden or on the patio, an easy job. You’re excited about test-driving it… so excited that you over-cook the fire, creating a massive blaze and using far too much wood. And, lo and behold, the blasted thing cracks. It happens, and it’s distressing.

About thermal shock – and how to avoid it

Thermal shock is a result of a clay chiminea being over-fired, when you use too many logs and/or create a large fire too soon. It has absolutely nothing to do with the quality of the clay chim you bought. It’s a clay thing, it can happen to anyone, and it makes a whole lot of sense to do everything you possibly can to avoid it.

That’s why we recommend making a careful, steady start with a clay chiminea, setting very small fires at first and working your way gently up to the full Monty. It’s the only real way to prevent thermal cracking, or at least lessen the likelihood.

What are the signs of thermal shock?

The tell-tale signs you’ve got thermal shock and have over-fired your chim are:

• Vertical cracks along the length of the chimenea
• A ping or cracking sound when it happens – you can’t mistake the sound for anything else
• If it’s a decorated chim, the paint goes very pale and bubbles
• You get thick soot marks around the mouth and chimney

Can you repair a cracked chiminea?

Luckily, yes, as long as the cracks aren’t too big, in which case it’s dead. You can carry out DIY repairs on small cracks easily and quickly using special fire cement – which we sell – and keep using your chim gently. Sadly the structure is compromised by the cracking, so it will never be as good as when it was whole and you’ll need to take extra care with it in future.

Patience is a virtue – Avoiding thermal shock

• Never over-fuel – if you get flames coming out of the top, your fire is way too big
• Clay chimeneas act like radiators. They gently radiate heat and there’s no need for a big fire – one or two logs is enough
• Avoid thermal shock by avoiding dramatic temperature variations over a short time, both heat and cold
• Before lighting you chim add 50 mm (2 inches) of lava stones, sand or gravel to protect the base from direct heat
  Kick off a small fire with balled newspaper and dry kindling or the special sticks we sell
• Never use petrol or spirit, which can easily cause cracking because of the sudden, intense heat
• Make two or three small fires first to cure the clay. This gradually makes the material harder and better able to resist thermal shock
• As a general rule, the more you fire it up, the harder the clay gets
• Always let the chim heat up gradually, running a small fire for at least 20 minutes before adding more wood or charcoal
• Even when you’ve successfully cured your clay chim, you still need to take care and warm it up steadily, not quickly
• Don’t add fuel via the funnel
• Bear the outdoor temperature in mind. If it’s very cold, it’s more likely the temp[temperature difference between hot and cold will cause cracking
• Always remove the lid when firing it up. Otherwise the chim will get really hot and crack
• Don’t put fires out with water – the temperature change will cause cracking

Remember thermal shock isn’t an age thing. It can happen however old or new your chim is, no matter how many times it has been used. It can even happen the first time you fire a new one up. But there’s one sure-fire way to free yourself from the risk of thermal shock, and that’s buying a chim made from Chimalin AFC clay.

Chimalin AFC clay – Incredibly tough and heat resistant

We worked hard for eight years to find a solution to the thermal shock issue. The solution is CHIMALIN AFC, a clay so sturdy and heat resistant that it simply doesn’t crack under heat. It looks like ordinary clay, but it performs so much better. And we stock a range of gorgeous Chimalin AFC chimineas.

Chimaling AFC is different to the other clays. Its formula is a secret, arrived at after 8 years of trials and study by Gardeco and a challenge to achieve. The clay from South China is normal terracotta clay, with added broken and ground-fired seconds. It isn’t really suitable for fires. On the other hand local lake clay from Mexico is ‘loose’ in its structure, allowing for better natural expansion. We added a secret refractory component to good quality Mexican clay which makes it more fire-resistant, and the result is our Chimalin AFC clay.

Chimalin AFC does crack under some circumstances and while it can’t be guaranteed against thermal shock, it is a whole lot more heat resistant than the clays usually used for chims.

As part of our testing procedures, we subjected one chimenea to repeated thermal shocks by making a huge fire then quenching it with water. We did this 20 times in a row and the chim didn’t crack. We also soaked a chimenea in water, left it to freeze and fired it from frozen, and it didn’t crack despite the fact that we carried out the same test three days running. And we kept one chimenea burning continuously for 2 months with coke – the fuel used to make steel – and it still didn’t crack. Even if small fissures appear on the surface it won’t result in material failure or a split body, something previously unheard of.

The superiority of our special clay is impressive. On a scale of 1 to 10, South China clay is 1, Mexican 3 and Chimaling AFC scores a nine. It’s still clay, though, and can crack if you knock or drop it.

Any questions? Give us a call and we’ll be glad to provide answers…