Once constructed, a new iron furnace — or one that has been "blown out" and allowed to stand cold — must be "blown in" before it is ready to smelt iron. In other words the whole furnace from tunnel head at the top to hearth stone under the crucible must be thoroughly dried out and hot. To accomplish this, the fillers who work in the charging house fill the furnace with charcoal and set it afire. When it has burned its way down into the bosh, the fillers refill the furnace again with charcoal. Once more the charcoal is completely burned, leaving in the bosh a bed of glowing, white-hot coals. During this lengthy burning any moisture in the furnace is driven out and its interior is now a fiery inferno ready for use.

"Blowing in" a newly built blast furnace was an infrequent and celebratory event. Often the iron master was there with his children and a number of workers, launching what they all hoped would be important to their incomes and welfare for years to come. Unfortunately "blowing in" happened fairly often when, for one reason or another, the furnace had to be shut down. Long intensely cold weather could lead to "blowing out" (shutting down) the furnace. So could a threat of water flooding the furnace and causing an explosion, or a failure of demand with no orders to be filled. Sometimes it seemed a furnace was, as one iron master put it, "almost as sensitive as [a person's] stomach." Improper heating during the blowing in, chilling caused by blockage of the blast, irregular filling, or overloading, "would soon upset it." Records of one furnace showed that one year it was "blowed out" five times (January 7, April 6, May 13, October 3 when a new hearth had to be installed, and November 12). The next year it "stopped up" on January 5, blowed out on March 31, May 22 and August 26, and "stopped up" on November 29 and December 27. Although the term "blow out" was usually used, most of these interruptions were not deliberate "blow outs" for major repairs such as relining the furnace with firebricks. Rather they seem to have been short term "upsets" that were only bothersome delays.