Harrow laid the new courses on the south wall with the stone he had traded for the bog-iron, and the mortar would not hold.
He knew why. He had known it since the first course went down, when he had pressed the trowel into the bed and felt the lime give under the blade like wet bread instead of setting like a bone. The iron pins were the answer. A pin of Forge iron, driven into the joint between two courses, took up the load and held the stones against one another, so that the wall stood as one piece instead of a heap of stones pretending to be a wall. The pins were what made a course true. Without them the mortar was only paste, and the paste would sag, and the stones would settle into the sag at the angle the weight wanted, and the wall would rise crooked and ugly and waiting to fall.
The Forge had no iron to spare for pins. Gaunt had said so in the Chamber, and had objected to it, and then had come down to the Mason’s Yard himself and looked at the empty bin where the pins had been kept and had said nothing at all, which was worse.
