The hardening and tempering of springs whose coils are of thick cross-section is performed at one operation as follows: The springs are heated in the furnace or oven described, and are first immersed for a certain period in a tank containing fish oil (obtained from the fish “Moss Bunker,” and termed “straights”), and are then removed and cooled in a tank of water. The period of immersion in the oil is governed solely by the operator’s judgment, depending upon the thickness of the cross-section of the spring coil, or, in other words, the diameter of the round steel of which the spring is made.
The table below gives examples of the hardening and tempering in this way of springs of the following dimensions:—
| Number of coils in spring | 5 | 3⁄4 | |
| Length of the spring | 6 | inches. | |
| Outside diameter of coils | 4 | 3⁄4 | |
| Diameter of steel | 1 | ||
| Examples. | Time of Immersion in Oil. | Number of Swings in Oil. |
| Seconds. | ||
| First | 28 | 35 |
| Second | 36 | 46 |
| Third | 27 | 36 |
| Fourth | 38 | 40 |
As will be seen, the spring in the first example was immersed in the oil and slowly swung back and forth for 28 seconds, having been given 35 swings during that time. Upon removal from the oil the spring took fire, was redipped for one second, and then put in the cold water tank to cool off.
The following are examples in hardening and tempering springs of the following dimensions:—
| Number of coils in the springs | 6 | ||
| Length of the springs | 9 | inches. | |
| Inside diameter of coils | 3 | 1⁄4 | |
| Size of steel | 1 × 1 | 1⁄2 | square. |
| Examples. | Time of Immersion in Oil. | Number of Swings in Oil. |
| Seconds. | ||
| First | 9 | 12 |
| Second | 8 | 12 |
| Third | 8 | 12 |
| Fourth | 9 | 12 |
| Fifth | 9 | 12 |
| Sixth | 9 | 12 |
To keep the tempering oil cool and at an even temperature, the tank of fish oil was in a second or outer tank containing water, a circulation of the latter being maintained by a pump. The swinging of the coils causes a circulation of the oil, while at the same time it hastens the cooling of the spring. The water tank was kept cool by a constant stream and overflow. If a spring, upon being taken from the oil, took fire, it was again immersed as in the first example. Resin and pitch are sometimes added to the oil to increase its hardening capacity, if necessary.
The test to which these springs were subjected was to compress them until the coils touched each other, measuring the height of the spring after each test, and continuing the operation until at two consecutive tests the spring came back to its height before the two respective compressions. The amount of set under these conditions is found to vary from 3⁄8 inch, in comparatively weak, to 7⁄8 inch for large stiff ones.