Before the welding work is begun a furnace periscope is used to determine the extent of the damage so that the exact work needed can be agreed with the customer. The periscope can be used to provide views into difficult corners so that the work can be carried out by reference to the monitor if necessary. After the work is completed it can be used to present the results obtained. Pictures or videos taken at regular intervals can provide the customer management with an appraisal of the furnace condition over the complete life of the furnace.