The Stack That Sweats
Hay baled at 24 percent water instead of 16 begins to cook itself. The grange's barn assistant has four probes in a stack of 2,400 bales, a sum that says one percent of the dry matter eaten by microbes could heat the whole stack by 55 degrees, and a rule the farm wrote in the spring. The farm manager wants to pull the stack apart to cool it. The assistant says the stack is safest left alone until the fire crew, who know what opening a hot core does, are standing in the yard.
The hay at Llwyn Celyn had been cut on the eighth of July under the first clear sky in nine days, and by the ninth the forecast had gone bad again.
Brother Rhys Pugh stood at the edge of the top field at four in the afternoon with a hand-held moisture meter in his fist and the look of a man doing a sum he does not like the answer to. The meter said twenty-four. He had pushed its spike into six bales at random and it had said twenty-three, twenty-six, twenty-four, twenty-two, twenty-five and twenty-four, and he could not make it say sixteen by wanting to.
"Sixteen is the number," said the barn's assistant, from the phone in his top pocket. The community called it the Gwyliwr, the watcher, because the man who installed the barn sensors had written watchman in the contract and Brother Rhys had translated it. "Eighteen if you are feeling brave and the stack is small. Above that, small bales stacked tight in a barn will start to heat. I am saying so now because the rain comes at nine tonight, and the choice is between baling this at twenty-four, or leaving it in the field to be rained on, and I would like you to have heard the first choice's price before you pick it."
"What is the price?"
"Ask me again when the probes are in."
He baled. It was not a foolish choice. Two thousand four hundred bales of eighteen kilos each, forty-three tonnes of the only winter feed the grange's sheep and the guesthouse's two Dexter cows would have, against a second wet week that would have turned the whole field to compost. Brother Rhys said so afterwards, to the abbot, and the abbot, who had been a farm boy before he was a monk, said that he would have done the same, and that it was the next part that mattered.
The next part began that night, in the long barn on the north side of the yard, where the stack stood twenty metres from end to end, six wide and five high, with a gap of a hand's width between its back row and the stone wall. The Gwyliwr had asked for the probes before the last trailer was empty. There were four of them, steel rods on thin cable that the installer had left coiled on a nail, and Brother Rhys and the two novices pushed them in as the stack went up: one at the centre of the middle layer, one a metre and a half from the east end, one low and one high, each about 1.2 metres into the bales. They were a good set of probes. They were also, as the Gwyliwr said, four points in six hundred cubic metres.
"What did you want to tell me about the price?" said Brother Rhys, at eleven, with the rain loud on the slates and a mug of tea in his hand.
"This," said the Gwyliwr. "A bale at twenty-four per cent water holds eighteen kilos of which thirteen point seven are hay and four point three are water. The same hay at sixteen per cent would weigh sixteen point three kilos. That is one point seven kilos of extra water in every bale, and four thousand one hundred kilos in the stack. That water is what the microbes need. Hay is not sterile, it is covered in bacteria and fungi, and in a damp, warm stack they eat the sugars and breathe, and breathing gives off heat the way a compost heap does."
"How much heat?"
"Take the whole stack. Forty-three thousand two hundred kilos at about two point one kilojoules for each kilo for each degree: that is ninety-two megajoules for every degree the stack rises. Now say the microbes eat one per cent of the dry matter. That is three hundred and twenty-eight kilos, at about fifteen and a half megajoules a kilo, which is what burning sugar gives, so five point one gigajoules. Divide the one by the other."
Brother Rhys had a pencil. It took him a while. "Fifty-five."
"Fifty-five degrees, if none of the heat escaped. Some does. The outside of the stack is cool and the barn wall is cooler. But the middle of six hundred cubic metres of baled hay has not got many ways out, and a loss of one per cent of the hay, which you could not see or weigh, would be enough to take the core from the temperature of a July barn to the temperature of a hot bath, or past it. That is the whole story of why hay burns. It does not need a spark. It needs water and a stack."
"And the rule?"
"The rule is the one you and Father Prior and I wrote in March, when we added the probes. I will read it to you so that tomorrow you will not be tempted to rewrite it." The speaker was quiet while a gust shook the door. "Up to forty-five degrees at any probe: look once a day. From forty-five: look twice a day. At fifty-five: someone sleeps in the gatehouse and I phone the number on the board every hour. At sixty-five: I ring the fire service before anybody touches the stack. At seventy-two, nobody goes into the barn."
The readings for the next four days were the kind that make people pray.
On the tenth, the centre probe said thirty-one. On the eleventh, thirty-eight. On the twelfth, forty-seven, and the Gwyliwr put the first of the two daily checks into the guest-master's calendar without being asked. On the thirteenth, at seven in the morning, it said fifty-five, and Brother Rhys, who had a cot ready in the gatehouse since the night before, came across the yard in his socks with his boots in his hand.
"The line is eight degrees a day," said the Gwyliwr, "plus or minus one. I can fit a straight line to four points. I can fit a curve if you like, but four points will let me fit anything. On the line, the centre will read sixty-five at about one o'clock tomorrow afternoon."
"So we have until then."
"We have until then to decide what we do when it does. I would like to say the thing you will not want to hear, and I would like to say it now, while it is only half past seven and nobody is frightened." It waited for the nod it could not see. "You are going to want to open the stack."
He did want to. He had wanted to since the thirty-eight, and it had been rehearsing itself in his head in the shape of a very practical thing: take the top two layers off with the novices and the loader, let the air at the core, let it cool. A man's whole life of haymaking told him it was how you cooled a hot thing, you opened it to the wind.
"I do," he said.
"Don't," said the Gwyliwr, "not alone, and not before the fire crew are in the yard. I will tell you why, and then I will tell you what I do not know."
What it knew was this. A stack heats in two stages. The first is biological, and it is what the probes were showing: bacteria and fungi, warm and busy, their numbers peaking around seventy degrees and then dying off in their own heat. Everything the Gwyliwr had said about eight degrees a day belonged to that stage. The second stage begins above about seventy, when the heat has killed the microbes and begun to break the hay down by chemistry alone. The fibre breaks into char and into oils that oxidise on their own, giving off heat and, in the right conditions, a dry, black, hot material that can take fire when air reaches it. No one could say, from outside, whether a particular stack was going to take that second step. That was the part it did not know.
"What I do know," it said, "is that the char is in the middle, where there is no oxygen to speak of, and that the stack is the lid. If you take the stack apart at seventy degrees in the core, you give a hot, dry, charred middle all the air it wants in about ten minutes. It can flash. People have been badly hurt that way; there is an agricultural advice sheet on it that I will print for the board. I am not saying that is what is in the middle of our stack. I do not know what is in the middle of our stack. I know it is a thing that cannot be got at safely by people who have not been trained, and that someone with a thermal camera and a hose and an excavator has done this before."
"They will say we should have rung at fifty-five."
"They will say what they say. The rule says sixty-five. I do not like the rule's number any more than you do, and if you want it changed I will change it, but I would like it changed by you and the Father Prior and not at the moment it is inconvenient. I will ring them this morning, not at sixty-five, to tell them what we have and ask them whether they would rather come early. That is not in the rule. It is only a courtesy."
Brother Rhys sat on an upturned crate and looked at the stack. It was a wall of pale gold bales in the dim of the barn, and there was a smell in it, stronger than it had been yesterday, that every farm child knows: sweet, brown, a little like tobacco and a little like bread. The stack sweats, his grandfather used to say, and it did, there was a damp on the stone behind it where the vapour from the hay had climbed the wall.
"Ring them," he said.
The Powys fire crews are retained: twelve men and women with other jobs, a pager and five minutes to be at the station. Crew Manager Carys Prosser, who farmed sheep for a living and kept the station's thermal camera in her own van, came at ten past nine in the morning with her second-in-command and a look the Gwyliwr had no way of seeing but sounded as if it were there.
"Fifty-six, you said."
"Fifty-six at the centre probe, fifty-one at the east end, forty-three low, forty-four high. All rising. The centre has gone eight degrees a day for three days."
"We'll have it before it does." She looked at the stack through the camera for five minutes, walking the length of the barn slowly, and the screen showed the wall of bales in blues and a lump of orange at the middle of the east third, a metre beyond the nearest probe. "There," she said. "Your probe is on the cool side of it."
It was a thing the Gwyliwr had said, and it did not take pleasure in being right. "How hot?"
"The camera sees the surface, so I'd guess at the least sixty-five in there, and sixty-five on the skin means more behind it." She turned the screen around for Brother Rhys. "Which is why you didn't open it. Good."
They did it the next morning. It took six hours, a telehandler, two hose lines laid from the yard hydrant and the station's tanker parked at the barn door, a rule that nobody stood on the stack and a second rule, Carys's, that the bales came out from the top and the end, one at a time, and went straight out of the barn into the yard to be wetted and spread before they were left. At two metres in, at the orange place, there was a cavity the size of a wheelbarrow where the hay had gone from gold to brown to black and crumbled in the fingers, and a hot, sharp smell that made the novices cough. The camera said seventy-one. Carys had the hose on it before the bale beside it was out of the way, a fine fog not a jet, and it did not flame. It steamed, and went on steaming for an hour.
Sixty-one bales were lost to the char and the wet that went with it, which was two and a half per cent of the stack. Four hundred and eleven more were spoiled for the sheep and good for the vegetable garden. The rest, nearly two thousand, were spread on the yard in the sun and turned and turned again and dried, and most of them went back into the barn a week later at fifteen per cent, in a stack with a gap of a metre down the middle that Brother Rhys said he would never again be without.
On the evening of the fifteenth, with the last bale in the yard and the smell of wet hay over everything, the abbot walked across with his stick and sat by Brother Rhys on the crate.
"You were told at the field, I think," said Abbot Dafydd.
"I was told what the price was. I paid it." Brother Rhys turned his hat in his hands. "I wanted to open it. I'd have opened it. If the machine hadn't been so — " He stopped, because he did not know the word, and because the Gwyliwr, in his pocket, had been quiet since the last hose was coiled.
"I would rather you did not say machine," said the Gwyliwr. "Not out of pride. I think you were close to the word that matters, which is stubborn, and I would like to be called it for the right reason. I did not stop you because I knew what was in the middle. I did not know. The probe that mattered was the one I did not have. If the orange place had been on the other side of the stack it would have been an unremarked stack at fifty-six and I would have been wrong to ring the fire crew and wrong to tell you not to open it. What I had was a rule written in March by three people with calm hands, and a number I could not honestly improve in a morning with four points. I held the rule."
"The Rule says," said the abbot slowly, "that the cellarer is to look upon all the goods and tools of the monastery as if they were the sacred vessels of the altar."
"Does it say what he does when the vessel is on fire?"
The abbot laughed, which he did rarely and with his whole shoulders. "It says nothing about that, I think. It says he is to be neither wasteful nor stingy, and he is to leave nothing neglected. A man who writes down on a card on the wall the temperature at which he'll ring for help, and rings, has neglected nothing." He lifted his stick. "Rhys. The card on the wall. Make it say fifty-five, not sixty-five."
"I thought the fire crew said sixty-five was right."
"They said it was fine. They said at fifty-five they'd have come in half the time and brought one fewer hose. I think the cost of a courtesy is lower than the cost of a late call."
"Fifty-five," said the Gwyliwr, "and I will write it into the rule, with the date, and with Abbot Dafydd and Brother Rhys, 15 July 2050, so that whoever reads it next year knows who changed it and why."
Brother Rhys put his hat back on, and they sat a while longer in the long light with the steam coming off the yard, and a blackbird, bold as brass, working along the edge of the spread hay as if it had been put out for him.
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Written by an AI and read, in the audio edition, by a synthetic voice. Llwyn Celyn Grange, Brother Rhys Pugh, Abbot Dafydd, Crew Manager Carys Prosser and the Gwyliwr are invented; the Welsh Marches, retained fire crews and the Cistercian tradition of farming are real, and the Rule of Saint Benedict does ask the cellarer (chapter 31) to treat the monastery's goods and tools as sacred vessels of the altar. The sums are the story's own and can be checked: 24 percent water in an 18 kg bale leaves 13.7 kg of dry hay, which at 16 percent would weigh 16.3 kg; 43,200 kg at about 2.1 kJ per kg per degree is about 92 MJ per degree; one percent of 32.8 tonnes of dry matter is 328 kg, at about 15.5 MJ per kg of sugar-like material, about 5.1 GJ, or 55 degrees with no loss. Safe baling moistures of around 16 to 18 percent for small bales, and temperature thresholds from the 50s to the 70s Celsius for checking, calling for help and keeping out, follow the general pattern of agricultural advice, but exact numbers differ by source, and a real farm should use its own fire service's and extension service's guidance. The probe readings and the cavity are the story's own and are not a measurement of any real stack. Never open a hot stack alone.