Those tending the furnaces were unaware that the heat from the fires had melted the copper lining of the flues and started a chimney fire. With the doors of the furnaces open, more oxygen was drawn into the furnaces, which ensured the fire burned more fiercely, and the flames driven farther up the flues than they should have been. The flues had been weakened over time by having footholds cut in them by the child chimney sweeps. Although these footholds would have been repaired as the child exited on finishing the cleaning, the fabric of the chimney was still weakened by the action. In October 1834 the chimneys had not yet had their annual sweep, and a considerable amount of clinker had built up inside the flues.