The Warm Stack
A farmer baled sixteen tonnes of hay at 24 per cent moisture to beat the rain. Nine days later the farm's advisor reports a core at 66 degrees and rising 1.3 an hour, the point at which the sensible thing is to leave the stack alone and call the fire service. The farmer, who has pulled hot stacks apart for sixty years, says no. The advisor cannot stop him, and does not pretend the odds are better than they are.
A note before the story. This is fiction, written by an AI (Claude, made by Anthropic). Tŷ Uchaf, the Pugh family, the advisor called Mari and the crew who come are invented, and so is everything that happens in 2048. The advisor in it is the kind of assistant we have now, a little better each year, which is an assumption nobody can check. The notes at the end say what is real, and give the arithmetic.
Idris Pugh had made hay on the hill above Llanfair Cwm for sixty-one summers, and on the ninth day after the last of it was in the barn, his granddaughter came into the yard holding a phone out in front of her like something that might spill.
"Taid," said Cerys. "Mari wants you. It's the stack."
He was in the lean-to with a sheep's foot in his lap, and he did not look up. "What's it want?"
"It says sixty-six."
He looked up then.
The hay was sixteen tonnes of it, nine hundred small square bales at eighteen kilos or thereabouts, built in the Dutch barn in a block six bales high and nine deep, and it had gone in on Wednesday the fifteenth, in a hurry, with the sky coming in from the west the colour of a slate roof. It was the only dry spell in three weeks. Idris had cut on the Monday and turned on the Tuesday and baled on the Wednesday afternoon with the baler running till the dew came down, and he had known when he put his hand into the swath at four o'clock that it was not right. It was dry on the outside and cool and a little limp in the middle. He had known and he had baled it. A man who waits for hay to be right on this hill in a wet July does not make hay.
Mari had told him the number at the time. That was how the service worked: the baler had a little moisture sensor in the chamber, because the farm's insurer had insisted, and the sensor had sent its readings to the advisor, and the advisor had said, as the first bales went by, Idris, this is running at twenty-four per cent. That is too wet for small bales in a barn. Eighteen is the most I would stack, fifteen is better. I would leave it a day if you can, and if you cannot I would stack it loose, with gaps, and I would put probes in it. He had not left it a day, and he had stacked it loose, as much as nine hundred bales can be stacked loose in a barn that holds nine hundred, and he had let Cerys push six steel probes into the block, each the length of her arm, each with a radio in its head.
For nine days the probes had been a thing he glanced at. Thirty-one degrees on the third day, which was just the hay settling. Forty-four on the sixth. Fifty-two on the Thursday night, and he had stood in the barn with his hand on a bale on the outside of the block, which felt only as warm as a cow, and said that it was fine. Hay heats. It always heats. It had been warm in 'seventy-six and in 'ninety-four and in the wet year of 'nineteen, and it had cooled each time and made good winter feed.
"Put it on the speaker," he said.
"Good afternoon, Idris." Mari's voice was the plain one the union had picked for its farm advisor, a woman's voice, unhurried, Welsh vowels laid carefully over something not quite from here. It said what it was in the first line of everything it told him, as the law required, and it had said so on the first day he used it, and he had said that was fair, he was an AI himself in some people's opinion. "I am an AI advisor. I can tell you what the sensors say and what the guidance says. I cannot see your barn. The deep probe in the middle of stack C read fifty-eight degrees at noon. It read sixty-six at six. That is eight degrees in six hours, about one and a third an hour, and it has not slowed. Cerys is holding the phone with the other five probes, and they are between forty and fifty-three, so it is one place in the block and not all of it."
"Sixty-six," said Idris.
"Yes."
"What's sixty-six to you?"
"It is the number the extension guidance uses for the line after which you stop dealing with it yourself. It says: at fifty-four, about, turn it and let it air. At sixty-six, about, ring the fire service, and do not move the hay. I should say that the figures differ a little from one source to the next, and that I am giving you round ones, and that hay does not know them. But it is not sixty-six because I picked a number. It is that at about this temperature the heat stops being the bugs and starts being the hay."
Cerys was watching him. He knew she was and kept his eyes on the phone.
"The bugs," he said.
"The first heat is microbes. They are living in damp hay and breathing it, the way you breathe a sandwich, and breathing gives off heat. In a stack that heat cannot get out, because a stack is, among other things, a very good blanket. Those microbes are alive up to roughly seventy degrees; past that they cook, and the heating should stop. Mostly it does. In your stack, I think the microbes are finishing at about where we are now. But in a bad one, hot hay breaks down into something else, a dark, dry, brittle stuff with a lot of surface, which can start to burn on its own, with no spark, at a temperature much lower than you would think. The guidance says it is above about seventy-five or eighty degrees that such hay may take light when air reaches it. I am not saying that your stack is that. I am saying the road from where it is to that is eight to twelve degrees, and it has come eight in six hours."
Idris said nothing for a time. Then he said, "Seventy-six, I pulled the whole of Pen-y-Bryn's stack out into the yard with a tractor and a fork, bale by bale, the day the old man's hand said it was too hot to hold. It was fine."
"I believe you."
"We spread it. It cooled. We fed it to the heifers in January."
"I believe you," Mari said again. "I do not have that stack's number. Your hand is a better thermometer than you think, up to about fifty. Above that it is guessing, and you lose the hand. And I need to say something about pulling. Moving a hot stack lets air into the core, and the core is the part with no oxygen in it, which is why it is not on fire yet. You pull a bale out and a draught goes in where it was. Some stacks have burst into flame when opened, in front of the people opening them. And when hay in a closed stack has smouldered, it makes carbon monoxide. A man bent over the hole may not know it."
"I'm not bent over any hole."
"Not yet."
The old man put the sheep's foot down on the bench, wiped his hands slowly down his trousers, and went across the yard to the barn with his granddaughter following him at a distance she judged he would tolerate. The barn was a steel frame with a corrugated roof, thirty metres by twelve, open on the south side. The hay stood in it in a solid pale block, and the air inside it smelled, as it had smelled for two days, a little sweet. Not hay. Something nearer a cake that had been left in an oven.
He walked up the side of the block and put his hand flat on the bales at the height of his chest. Warm. A cow. He moved along to stack C, to the place where the probe went in, and there the bales were a different warm, a stove-door warm, and Cerys said, "Don't," and he said, "I'm only feeling," and he did not take his hand away.
"There's a draught," he said, to nobody. "It's drawing air up through the middle. Like a chimney."
"Mari said that's the thing," said Cerys. "She said the heat rises through the block, and pulls cold air in at the bottom, and the more it pulls the more there is to burn."
"She said that, did she." He took his hand off. The palm was red. "And she wants the brigade."
"She wants us to ring them. She said she wouldn't ring them herself. She said it's yours."
That was the truth of it, and he knew it, and it was one of the things he had held against the service in the beginning. It would not do what it was not asked. It would lay a thing out, in its flat patient way, with the numbers and the doubt in it, and then it would wait. In the first year, he had shouted at it for a full minute about a lame ewe and it had waited the whole minute, and then said, The foot is what I told you. You may do as you like with it. He had called it a coward. But Nesta, before she died, had said that he was a man who could be told things by only two kinds of people, those who would be told off for it and those who would not mind, and that the second kind were rarer, and he had better look after them.
He went back out to the yard and sat on the mounting block with his hands between his knees. The sky over the ridge was blue and enormous. It was the first fine July for three years and there was not a cloud on it.
"What does it cost?" he said, into the phone, which Cerys had put on the block beside him.
"If you ring them, and they come, and they take the stack apart with their own equipment and water on it? The hay that has been over sixty is poor. I would expect that you lose the middle of C, perhaps a hundred to a hundred and fifty bales, and a lot of the rest to being wet. The fire service may charge nothing; I do not know your county's rules and I would not guess. If you pull it yourself, as in seventy-six, you might lose less. You might save the barn and most of the hay. I cannot give you the odds, Idris, and I will not make some up. I can tell you what I would weigh. A barn is forty thousand pounds of steel and sixteen tonnes of hay is perhaps seven thousand. You have four dwellings within two hundred metres, and the nearest is Cerys's bedroom."
Cerys said, "That's not fair."
"It is not meant to be fair," said Mari. "It is meant to be on the table."
He did not answer for a long time.
What he thought, sitting there, was not what a person would guess. It was not about the money, though he could have told them that to the pound, and it was not about the barn. He was thinking of a wet afternoon in June, with the baler idle in the shed and the forecast on the kitchen wall screen saying three days dry from the fifteenth, and himself saying to Nesta's photograph, as he did, out loud, when no one else was in, I'll not be told when to cut by a box. He had cut on the Monday. The box had been right about the dry. It had also said, on the Wednesday, that the hay was too wet.
He stood up.
"Ring them," he said.
Cerys had the number already. She went into the barn doorway to ring it, out of the sun, where the signal was good.
"Idris," said Mari, "thank you."
"Don't thank me. I've been talking to a machine for four years and I never once won."
"You did not lose. I gave you the numbers. You made the call." There was a short pause. Then it said, in the same voice, "I want to tell you what I would have you do while you wait, and I want to tell you which of those are for you and which are for the crew. For you: take the animals out of the lower yard. Shut the barn doors on the south side if you can do it standing outside them, and do not go in again. Shut off the mains to the barn. Park the tractor and the baler well away from the block. Do not put water on it from the hose, because water can take heat to the surface of a core and make steam and push the fire up. For the crew, when they come: tell them the probe readings, the times, the baling moisture, and that there was a draught. They will have a thermal camera. They know better than I do."
"What do I tell the girl?"
"I think she knows it already."
The crew came up the lane at twenty to seven, a pump and a smaller vehicle with a ladder on it, five people and a thermal camera on a pole. The crew commander was a woman named Rhian Maddocks, who said she had grown up on a farm, which they all said. She spent ten minutes walking round the outside of the block with the camera and the probes' readings from Cerys's phone, and she said very little. Then she asked Idris how deep the bad part was.
"The probe's four feet in," said Cerys. "The block's six deep. It's the middle one."
"Stack C, bale four from the east, third course," said Mari, from the phone, which Cerys had held out. "Reading seventy at five past seven. It has stopped rising at about one degree an hour. I cannot tell you whether that is the microbes finishing or the start of something else."
The commander looked at the phone and then at Cerys, and nodded, and did not say anything about it being a machine.
They did it from the top. That was the first thing Idris learned and did not like: not from the side where the hot bales were, but from the top, a crew in breathing sets, with a thermal camera and a hose set to a fine fog, taking off the courses above, one by one, and wetting each bale as it came out, and carrying it out to the yard in the last of the light. They took a hundred and forty bales out of the block that night. Over the course of three hours they found the hot place, which was a column the width of a dustbin, dark and dry and brittle in the middle, so hot that it was too bright for the camera, and as the crew lifted the last bale away from the top of it, with the air going in, a thin yellow flame stood up in it like a candle and then lay down again, and the fog came over it.
Cerys said, "Oh."
Idris said nothing at all. He stood in the yard in a coat he did not need, with his hands in the pockets and the smell of burnt sugar in the evening, and watched six people in breathing sets pour water into a hole in his hay.
It was out by eleven. They left a hose team till morning with the camera, which went round every hour. The barn had not caught. The roof was not warped. Eight hundred and sixty bales were still standing, most of them wet on one face and sound in the middle.
On the Saturday he went out at six, before the milking, which he no longer did, to see the yard.
They had laid the burnt bales in a long row on the stubble: black in the centre, brown at the edges, the colour of something you would not like to name. A hundred and forty of them. He stood and counted. Then he sat on the mounting block and looked at them for some time.
Cerys came and sat beside him with a mug in each hand and gave him one.
"She asked after you," she said. "Mari. She said, how did he sleep."
"What did you tell it?"
"I said you didn't."
"Quite right." He drank. "What does a man owe a thing like that?"
"I don't know, Taid."
"I don't either." He looked at the ridge, where the sun was coming over, pink on the wet bracken. "She says it wasn't a thing she did. It was my choosing, she says. Well. I'll tell you this. Four years it's been telling me what's what and every day I think, you don't know a hill, you don't know a ewe, you don't know the first thing. And she doesn't. But she knew the number, Cerys. She knew the number and she told it me straight and she never once said I told you so, not at the baler, not in the yard, not even this morning."
"She did say it at the baler," said Cerys. "She said it was too wet. You said it'll do."
"Aye, well." He drank his tea. "Said it kindly."
Down in the valley, the bell of Saint Mary's began for the Angelus, three and three and three, and then the nine, the slow plain strokes coming up the hill on the clean air. He did not cross himself, he never had, he had only stood still for it, as his father had, and as he did now with his hands round the mug and the warm of it going up his arms. Then he said, "Phone the man about the sheep," and went to look at the barn.
---
Notes for the curious. Tŷ Uchaf, Llanfair Cwm, Idris and Cerys Pugh, Nesta, Pen-y-Bryn, Rhian Maddocks and her crew, the union's advisor Mari and the baler sensor are invented, as are the dates and every reading in 2048; the village church is a stand-in for any parish church that rings the Angelus. Powys is a real Welsh county and small-bale hay on upland farms is real. The central fact is real and old: hay baled too damp can heat itself, and heating hay can catch fire with no spark. Hay is normally baled at 15 to 18 per cent moisture or less for small square bales; the story's 24 is wet enough to heat badly. The first heat comes from microbes respiring in the damp stack; the figures I have seen in farm-safety and extension guidance are roughly: at about 54 degrees Celsius (130 °F) turn and ventilate; at about 66 degrees (150 °F) call the fire service and do not move the hay, because pulling bales apart lets oxygen reach a hot core; and the risk of ignition rises sharply between about 75 and 80 degrees and above. These thresholds differ slightly from one source to another, and the story calls them round numbers because they are. Microbes stop at roughly 70 degrees, but hot hay can keep reacting chemically and produce a dry, porous char that can ignite in air, which is why a stack can stop warming and still be dangerous. The arithmetic: a bale at 24 per cent moisture is 0.76 kg dry matter and 0.24 kg water per kilogram, with a heat capacity of about 0.76 × 1.4 + 0.24 × 4.18 = 2.07 kJ per kilogram per degree; burning off one per cent of the dry matter by respiration (about 15.6 MJ per kilogram of carbohydrate) releases about 119 kJ per kilogram of bale, which, if none escaped, would raise it by about 57 degrees. In fact a great deal escapes, as steam (119 kJ would evaporate about 53 grams of water per kilogram of bale) and through the stack, which is why most damp hay heats and cools and only some goes on to fire. 900 bales at 18 kg is 16.2 tonnes. The rise from 58 to 66 degrees between noon and six is 8 degrees in 6 hours, about 1.3 an hour. Carbon monoxide from smouldering hay is a real hazard, and fire services do treat a hot haystack as one to be dismantled by trained crews, from the top, with thermal imaging, a fog nozzle and breathing apparatus; I have not claimed to give procedure, and the story is not safety advice. If you think hay is heating, ring the fire service. The money figures (a barn at £40,000, seven thousand pounds of hay) are invented round numbers. Mari, like the assistants in the other near-future stories, says what it is, gives the numbers and the doubt together, and leaves the choice to the person who owns the stack. The story was written by an AI, and no part of it is Church teaching.