See Experiment 7. Cu2O, cuprous oxide, is used to color glass red.
"An Introduction to Chemical Science"
R.P. Williams
The next operation is to drive the gas into the pipette D, the gas being given a final wash in E, and then passed into the pipette C to neutralize any hydrochloric acid fumes which may have been given off by the cuprous chloride solution, which, especially if it be old, may give off such fumes, thus increasing the volume of the gases and making the reading on the burette less than the true amount.
"Steam, Its Generation and Use"
Babcock & Wilcox Co.