When these plates are installed, whatever kind may be used, provision must be made for obtaining access to them. In Fig. 165 two platforms for this purpose will be seen above the platform at the level of the mantle, these platforms being provided with a suitable handrail. This is important because in case of trouble with the circulation in these plates it is necessary to obtain access to them quickly in order to prevent their being burnt, and this cannot be done if ladders have to be arranged for the purpose. Moreover there is always a certain quantity of furnace gas stealing up the walls of the furnace, no matter how well it may be constructed, and while this may be so dilute that the workman exposed to it will not realize the danger, he may nevertheless be made unconscious by it. Under such conditions if working from a ladder he is almost certain to fall and be seriously injured or killed, whereas if a secure platform with a good hand-rail be provided he will fall upon this and can easily be rescued before further poisoning by the gas.

The number of rows of cooling plates above the mantle is a matter of the individual conditions and the judgment of the operator. The customary number is probably from three to six rows, although in several cases these rows have been run all the way to the top of the furnace, spaced about as shown in Fig. 165. This is often done with furnaces intended to be run on ferromanagnese or spiegel which are highly destructive of their linings. For ordinary furnace practice the consensus of opinion is that there is no advantage in having more than a limited number of rows of these plates.

There is a drawback to the use of these plates in that while they prevent the lines of the furnace from cutting back at the plate they do not prevent heavy erosion in the space between the adjacent rows very much as shown for the bosh in dotted lines in Fig. 164 and the steps so formed offer a lodgment for scaffolds in the very region of the furnace where the stock is in its pastiest condition and therefore most likely to scaffold. Consequently these steps tend to promote scaffolds with all their disastrous consequences.

For this reason some operators prefer not to use cooling plates above the mantle at all, but to let the lining wear away and cool the shell externally with sprays if it becomes necessary. It is stated by Mr. H. A. Bassert in his paper on Modern American Blast Furnace Practice read before the American Iron and Steel Institute, May, 1914, that "cooling plates above the mantles have only survived where they are placed from 18 inches to 22 inches back of the face of the inwall and in this position they have not much effect in preserving the original lines".

This statement may not be universally true, but may be considered as absolute proof that cooling plates above the mantle are not a panacea for lining troubles but on the other hand are only to be used sparingly and with great discretion.

From this region at the base of the shaft to a point a few feet below the stock line conditions of the temperatures and abrasion are such that well-made firebrick alone will withstand a long campaign, and consequently in much modern practice nothing else is used.