A potato, baked in the oven or crackling over hot coals, exudes a pure, earthy aroma arousing deep memories of holiday dinners past. Or your first camping trip, cooking directly over dying embers. For me, the skin is the best part. After eviscerating the potato- the skin is oiled, salted and grilled- it's everything potato chips lack.
For many years, restaurant potatoes were wrapped in foil and more or less steamed- an abomination. Though an understandable short cut- a large spud might take an hour to bake, too long to be cooked to order. Today, if you are in a hurry, a quick 3-5 minute pass through the microwave, followed by 15 minutes in a 450F oven, will do the trick.
But an alternative method is to roast the potato on a spit. More precisely, by impaling the potato with a 1/4" diameter, six-inch long nail. The theory being the metal nail conducts heat deep into the center of the potato, and the in the words of one commercial, reduces cooking time by "up to 50%".
So I purchased a commercial spud nail kit. The spikes are made of food grade stainless steel1, and come four-to-a-set. I then baked a dozen 8 oz Idaho, Red Bliss and Yukon Gold potatoes in a 400F oven according to their instructions.
Did it work as advertised? Well, if you think 10% counts as "up to 50%", it does speed up cooking. But hardly worth the effort. Here's why:
I instrumented each potato with two very small thermocouples. One was located in the center of the potato, the other, mid-way between the center and the surface. In the case of the nailed potato, the center thermocouple was in the center of the spud, but immediately on the surface of the spike.
A potato is fully cooked at around 195F (as confirmed by poking with a narrow skewer). As you can see, while both potatoes cooked at the same rate at the midway points (with or without the nail), the center took about 5 minutes longer to reach done without the spike. Under whelming.
But the principle seems sound- why didn't it work as advertised? The reason is thermal conductivity, or lack thereof. Thermal conductivity "κ" measures how quickly heat travels down the nail, into the potato. Glass, an excellent insulator, has a low κ of 0.8 W/m-K. The potato is pretty poor as well, around 0.5 W/m-K. But stainless steel is only 18 W/m-K, compared to an excellent thermal conductor like copper (380 W/m-K)- almost 20 times larger than stainless! This is why high-end sauté pans embed a thick copper disk beneath their stainless steel skins. The copper spreads the heat, preventing hot spots and charring.
As you can see from these images of a potato cooked in a simulated oven, the nail's thermal conductivity makes all the difference (blue is cold, red is hot). The stainless steel spiked potato is barely warmer than no spike at all.
A copper nail (or more safely, a copper nail coated with a stainless steel skin, or even tin1 like in a tin can) might actually reduce cooking time in half. But not plain stainless, or even plain steel.
If you plan on roasting a meal over an open fire with a rotisserie, you might look for a spit made from a low conductivity metal. Otherwise, in a long cook the 1/2" thick spit can easily over-cook the food's interior, especially where it first penetrates. Stainless steel isn't a bad choice, but even better is a hollow rod- air is a 700x worse conductor than stainless.
But for potatoes? At home, if you are in a hurry, pre-cook with the microwave. Out camping? The whole point is to slow down and enjoy life. Leave the nails at home.