In an example implementation, a sintering system includes a detection gas line to enable gas to flow into a sintering furnace from an external gas supply. The system includes a detection gas port inside the furnace through which gas from the detection gas line is to flow into the furnace, and a registration feature inside the furnace to enable positioning of a token green object proximate the gas detection port. The system includes a gas flow monitor to detect changes in gas flow through the detection gas line when the token green object shrinks during a sintering process in the furnace.