Tuesday, January 11, 2011

Hot Potatoes under the Bedcovers

Photo from jcmedinas photostream

I'm sure we all have heard the stories of putting hot potatoes, a hot water bottle, or even heated bricks under the covers at the foot of the bed to warm our feet when going to bed in a cold room that was far, far away from the wood stove. Brrrrrrr!

I want to take that 'technology' a bit farther here and apply it to cooking. What happens with a solid heated object like potatoes or a brick (technically both are a heat source) wrapped in blankets is that the heat radiates out for a long time, heating whatever is nearby... in this case, feet. To the extent that it works is dependent on a couple of factors. 

One factor is the mass of the heated object: the greater the mass, the more heat it holds, at least initially. So, if you have 4 hot potatoes and your sister only has one... who will have warmer tootsies? Another factor is the insulation around the hot potato. If the covering blankets are just a layer or two of thin cotton, they will not insulate very well, nor for very long. Pretty soon your tootsies will be just as cold as the room. However, if the cover is a thick down comforter, the heat may last for hours, continuing to warm your tootsies the whole time.

Now, consider a pot of stew cooking on the stove... actually boiling. If you have kitchen duty, you would probably turn the heat down to a simmer, add a lid, and let the stew cook under very low heat for a few hours. In earlier days, you would need to add a bit of firewood from time to time to keep the heat going. Today we just pay for some additional BTU's to keep the pot warm, and those are being calculated (and rung up) by the meter outside, unseen for the most part but a cost nonetheless.

However, if you took that same boiling pot and put it under the down bed covers, it would continue to cook... for free! It might take longer to finish cooking, depending on how well insulated the bed covers are... but you get the idea.

Now take that concept of the hot pot under the bed covers just a step farther... Construct some sort of a box (cardboard will do as a start) with a lid and place it in the kitchen, but rather than using the bed covers, line the box and lid with some really good, thick insulating material... and then when the pot boils, place it in the immediately in the box. Turn off the stove, cover the pot with the insulated lid, and go about your business. Your stew will continue to cook!

In the early 1900's, merchants sold what they called "Fireless Cookers" with great monetary success... and those were nothing more than a well-insulated box of either wood or more often, pretty painted metal to hold a hot pot (or two side-by-side) for further cooking at low temperatures, without any additional source of heat.

Urban Haybox Photo from Eithin's Photostream; (Note heat reflection but lack of insulation)
Long before the 1900's, folks with limited cooking fuel used the same principle, lining boxes with hay they had on hand for insulation, and so they became known as a "Haybox". Hay is not the best insulation material we have available now, but it will still do in a pinch. Here's a page with more information on fireless cooking.

This same principle applies today in solar cookers; they work best with an insulating barrier to the air outside the container and box, which would otherwise wick away the heat. Counteracting that heat loss is usually accomplished by enclosing the air space around the cooker with insulation in the walls. The better the insulation, the less heat loss.

Photo from thoth on Flickr

You can use that same principle to cook a breakfast cereal grain overnight in a good vacuum thermos. Measure a portion of a cereal grain (I like steel-cut oats) into a wide mouth thermos bottle. Bring the appropriate amount of water and a pinch of salt to a hard boil and pour into the thermos just before bedtime. Screw the lid tight, shake a couple of times to mix the water and grain together well, and set aside until morning. It will be perfectly cooked when you awaken!

Note: Thermos vacuum bottles vary in how well they are insulated. The small stainless steel briefcase-size one I use to keep my coffee hot after making coffee in my glass Chemex drip coffee maker only keeps my coffee fairly hot for an hour or two. The Nissan Travel mug I carry in the car will keep my coffee steaming for several hours, or an iced drink cold (and often with ice intact) overnight.

If your vacuum thermos has thinner insulation like my briefcase thermos, you could wrap it in an insulating blanket (or even a thick towel) to keep more heat contained for overnight cooking. You'll just have to fiddle with what works best with what you have.

Photo from fr:Utilisateur:Nataraja

The oriental bento box, and the stackable food delivery boxes such as the one shown above and used by many cultures could accomplish much the same thing if they were insulated.

I can see endless opportunities to reduce my electric consumption by using fireless cooking. Something as simple as some dehydrated soup ingredients and boiling water placed in my thermos at breakfast time for my lunch...a pot of rice started at noon and placed in the haybox/fireless cooker (which I WILL I build soon) for my evening meal? A meat and vegetable stew?

It may take some trial and error to see what cooks best, and what design and insulation of a fireless cooker works best, and I think this might be a fun and challenging thing to do.


  1. Hi,

    Thanks for the photo! I just want to add, the side walls are actually made of three layers of cardboard (old packing boxes, in fact) wrapped with foil, not just heat-reflecting foil alone, and the combination is actually very efficient.

  2. Thanks for adding your construction details, Eithin! I made assumptions, sorry.


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