When Gendry and Thomas had followed the stone-kin and the young maester helper into the stone-kin caverns, the box was thrown into a forge. The box's metal didn't even discolor or turn red, but a small red light on one side began blinking. Then the red light went out, and a green light started blinking. Soon, a second green light started blinking, and the first stayed green.
When the third light quit blinking and all three lights were green, the stone-kin recognized that this box was very much like the one attached to the stone melter that the maesters still had, so they attached this box to the 'rented' melter they had. The whole group of them then marched to the room, and the stone-kin began to melt the oily black stone from a wall. They started in the entranceway and aimed the device at the first edge of the black stone. After a few minutes, the black stone sagged a little, and one of the stone-kin pushed it with his hammer. The hammerhead sank in an inch or so, and the stone-kin had to really pull to get his hammer back out. So they kept the device working on the stone until it started sagging off the wall. When it fell, one stone-kin stuck his iron forge bar into the glob before it could stick to the floor. One of them accidentally touched it, jerking back expecting to be burned, but the stone was barely warm. Once the glob stopped being sticky, they set it and the iron bar on the floor and moved to a hollow spot in the wall they thought was a door. The device hummed as it worked across the surface, and they slowly cleared a spot that was steel underneath. But the device would not melt the steel. So they hammered on the steel, hoping to smash the door open. It wouldn't budge. After several hours, they discovered the door had no latch and no hinges. Instead, it was mounted in a track. They removed the black stone from above the door and found pins driven through the base stone holding the door's upper guide. One forge master produced a chisel and hammer and drove the pins back into the space behind the door. With a hard shove, the door fell into the opening beyond.
After yanking the door free, they went inside and found two medium-sized devices. The things sat on wide bases with some scoop in front. They rested on what looked like a bracelet made of metal slats hinged together. The things had slender upper parts that looked vaguely humanoid, but they had no head or arms. The maesters in their nightshirts sent the young helper back to the maester building to get a wagon and some help. The wagon arrived, and they loaded the two heavy devices and carried them back to the building.
It was getting late, so the stone-kin took their glob of black stone back to their forges and began experimenting with it. They discovered that putting a melted part of it into any form would make it stick to that form. Gendry suggested they oil the form first so the cooled black stone would come out easily. They tried 5 or 6 oil types with no luck, but Thomas suggested they try fish oil, and it worked. The stone-kin got out several forms they had already made. Two of the forms were for a new design of war arrowheads. They had a jagged tip that would cut into armor better. They had to use the device on the stone long enough for it to run off the bottom of the glob and drip into the forms. They also filled a tray of 40 slugs, as Thomas was now calling the shots on their shoulder-gun loads. The stone-kin also filled a sword mold with the last of the glob. When these forms cooled, they had to bang them hard on their backs to get the finished pieces out. The sword was so heavy that only the stone-kin could pick it up. One of the forge masters said it was too heavy to use for armor unless they could make very thin plates and sew them into a gambeson.
It was well past dark by then, and Thomeas and Gendry took the slugs and a couple of the arrowheads with them as they went towards home. The next day they tested the slugs, and they fired fine and punched through heavy wood and armor without breaking up. But for actual damage to a living thing, the lead slugs did more damage as they flattened out when they hit something. The arrowheads, on the other hand, were deadly and chopped through whatever they were aimed at. The next day, they had Qotho test them, and he drove one through a dragon scale and the metal beneath it.
When Arya returned from clearing her head, Thomas was waiting for her. He said, "I'm sorry, my love. I didn't mean to be sharp with you. I just wanted you to understand that being a leader demands that you can send men to die if need be." Arya nodded her head and said, "Apology accepted." They went to bed together, but Thomas was unsure the matter was settled. As they were falling asleep, Arya quipped, "I will speak to my brother about this when he comes." Thomas hugged her and fell asleep.