There was widespread praise at the start of the summer for question 11 on the Leaving Cert Physics paper - or at least there was amongst the physics teachers I follow on Twitter.
It covered the physics behind various aspects of life in Bronze Age Ireland, including the use of the Fulacht Fiadh for cooking. As it happened, this was something I had read about just recently - while helping my teenager prepare for a first year history exam - and I thought it sounded like a great way of cooking. But the first year history book didn't get into the physics of it - and now that the leaving Cert has done, I'm not so convinced!
The question told us this:
A particular fulacht fiadh contained 750 litres of water at an initial temperature of 4 °C. 50 stones were taken from the fire, at a temperature of 280 °C, and placed into the water. The stones had an average heat capacity of 8.5 kJ/K each.
When we work through the details of the question, we are asked to find the temperature the water would have reached - and my calculations are below:
Heat lost (by the stones) = Heat gained (by the water)
CΔϑ = mcΔϑ
(8500)(50)(280-x) = (750)(4180)(x-4)
3,560,000 x = 131,540,000
So - if I have the above right - the temperature would only have been a little under 37 degrees when the meat was added (if it was meat that was being cooked). But what is going to cook at 37 degrees? And how long would it take?
I've checked the food safety authority website (fsai.ie) and they tell us this: Bacteria grow best in warm temperatures, approximately 25°C – 40°C. Lukewarm food is dangerous as it is a perfect environment for bacteria to thrive.
Furthermore they say to cook any meat that is likely to host bacteria at 75 degrees or more.
The bronze age cooks presumably weren't answerable to the FSAI, but presumably they were prone to bacterial infections. So - what with a bit of time to hand over the holidays - I did a little reading, and came across this research article: Fulachtaί fia and Bronze Age cooking in Ireland: reappraising the evidence on JSTOR - in which Alan Hawkes says (if I read him correctly) that its not universally agreed that Fulacht Fiadh were used for cooking. And that some sources argue they could have been used for bathing, brewing, metalworking, tanning, dying and/or washing. Furthermore, they might have been connected with the use of sweathouses.
If they were used for cooking, the article does seem to suggest that the water was boiled - which goes against the numbers we have in the question. But it's not hard to make things fit. One obvious variation in their use would have been to not fill them with water. If there was less water used - and 750 litres is a lot of water - the water that was there would obviously reach a far higher temperature - suitable for cooking. Indeed the chefs would have been able to choose almost any temperature they wanted up to 100 degrees - and could have maintained that temperature by continuing to add hot stones.
The article also refers to the roasting of meat, and of course the bronze age cooks may have made use of a variety of techniques: for example they could have initially roasted the meat on the open fire (which was obviously right there) before transferring it to hot water to continue cooking.
But personally I quite like the 'bathing' argument. The Bonze Age people might have been hardier than us, but it must always have been more pleasant - even for them - to bathe in somewhat warm water. 37 degrees is of course body temperature, and as a modern bathtub holds something like 300 litres of water, the 750 mentioned here would surely have been enough to satisfy our somewhat diminutive ancestors.