&oiDuring the Copper Empire’s peak, over 100 steam stamps were in operation across the peninsula. As mines and their mills succumbed, these massive stamps were quickly sold or scrapped for quick cash. With the arrival of the Second World War any remaining stamps were quickly drafted into the war effort – providing much needed steel and iron for military equipment. By the end of the empire in the 60’s, stamps had become and endangered species. Today only one has managed to survive.
Sitting high atop its massive concrete foundation, the iron monstrosity looking out over Tamarack City is one of eight stamps that once graced the massive Ahmeek Mill. While all of its brothers have been removed,this one has inexplicably managed to survive. As one of the last mills to close down on the peninsula, this stamp represents the best and brightest of Copper Country industrial know-how. Because of this, the Ahmeek stamp is a slightly different breed then what we had been discussing lately.
As far as I can tell, the stamp at Ahmeek is of a steeple-compound design – probably a Nordberg. This stamp is very similar in almost all regards to the Allis stamp featured previously except for the addition of a second driving cylinder powered by low-pressure exhaust steam from the main cylinder. This is known as a compound engine. When the two cylinders work the same shaft (in this case the stamp shaft) and are mounted on top of each other, the engine is then known as a steeple-compound.
To get a better idea of how this type of set-up works, we’ll take a tour of the process along the diagram of a Nordberg stamp seen above. High-pressure steam from the boiler house enters the first cylinder (B) by means of a double-acting valve assemble (A). Here steam is used to both push the stamp shaft (I) down, as well as push it back up (thus the double-acting part). After expanding in the cylinder this steam loses some of its heat and becomes low-pressure steam. The low-pressure steam is exhausted through (C) and enters a holding tank (D). From there the steam is send upwards through a connector pipe (E) before entering the low-pressure cylinder at (F). In this cylinder (G), the low pressure steam is used only to help push the stamp downward, (single-acting) and does not raise the stamp. The steam is then exhausted (H) and the process repeats.
Let’s take a closer look now in practice – using the remains of the Ahmeek stamp as a guide:
Compared to single cylinder stamps such as the Allis, these Nordberg’s are easily identified by the iron box sitting atop of the unit. This box houses the low-pressure cylinder and valve assembly. The high pressure cylinder sits directly below this. Both valve assemblies are controlled by a series of rods and cams, which are powered by the large pulley wheel on the left. This wheel would have been hooked to a belt drive from the ceiling, powered by a steam engine somewhere in the plant. Below the cylinders and valve drive is the stamp shaft and mortar box. You can make out the feed chute in which ore would have entered the mortar on the right.
While the stamp appears to be intact, it is missing the low-pressure storage tank and separator assembly as well as the low-pressure inlet pipe that would have connected that assembly with the upper cylinder. These pieces were probably removed in anticipation of scrapping the stamp, but for some reason the process was never finished.
Here’s a closer look at the upper cylinders and valve eccentrics.
Here’s a closer look of the Mortar assembly. In this view you can clearly make out the stamp shaft, as well as the attached pulley which would have been used to turn the stamp shaft every few cycles. Below the mortar box is the remains of the discharge chute (which appears to still have sand in it since grass can be seen growing out of it). From this chute the copper rock would have been sent on to the next step in the process.
A step we take a closer look at next…
Information and illustrations from “A Textbook of Ore Dressing” by Robert Richards.