How to make Soap and a Hot Shower on a Desert Island

How do you make soap and a hot shower on a desert island? In "Red Sand", survivors stuck on a desert island struggle with natives, sand fleas, a mysterious killer, and certainly hygiene issues. The mechanics of a hot shower would have slowed down the plot, but if you read it wondering, "How did they stay clean?", here's the answer.
Surrounded by stinky castaways? You're not the first. Among the travails of marooned mariners, hygiene typically gets the short shrift. It's hard for us modern folk to focus on famine, disease, and elemental exposure when the guy next to you stinks to high hell.
Luckily, even the rudest island has all the ingredients necessary to take a hot shower with real soap.
The following recipe
comes from the true story of survivors of the 1864 Grafton shipwreck in New Zealand as detailed in Joan Druett's book, "Island of the Lost: Shipwrecked at the Edge of the World". Highly recommended reading.

Soap ingredients:
Lye
Lime
Fat

Shower Ingredients:
Heat
Fresh water

How to Make Soap
Let's start by answering the question, "How do you make soap?" Man has made soap since the beginning of time. Just because we buy it at the store now is no excuse NOT to know how to make it. If your environment has food to eat, you can make soap.

Ingredients:
A pile of seashells
A big pile of dried seaweed (or any organic plant matter)
Three buckets or containers. You can make these from fired clay using much the same techniques as soap.
Animal Fat or grease

1. Place a pile of crushed seashells over a big pile of dried seaweed. You can also use wood or any plant instead of seaweed if you have it available.  Light the seaweed on fire and let it burn down completely. The result is a pile of all the ingredients you need. Soda ash (sodium carbonate) and potash (potassium carbonate) are naturally found in the ashes of the burnt seaweed. Lime is the result of heating the seashells to above 1000 degrees Celcius (1800 degrees Farenheit).
2. Drill holes in a bucket. Prop up the bucket with something like stilts or rocks and slide a another bucket underneath.
3. Using the third bucket, slowly pour fresh water over the ashes. The water will flow through the holes at the bottom, taking with it soda, potash, and lime.
4. Remove the bottom bucket. By now, the lime, soda ash, and potash are mixing together to form a solid. This is called Lye. Be very careful not to spill the contents of this bucket on yourself as it will cause a chemical burn.
5. Add animal fat to the water. It's best to use the grease of whatever you've been eating - mammals, fish, whatever. You could also add the fat of humans, ala Fight Club, if that's all you have to eat!
6. Bring the mixture to a boil until the water boils away. Stand back. It will stink like the unholy. At first, the animal fat will float. When it sinks to the bottom, it has been "rendered" into lard.
7. Strain off the excess water. When the lard cools - Voila! Soap!

A side note on vegetable oil.
If you're really hankering for some island luxury, use vegetable oil instead of animal fat. Coconut oil is great if you don't have olives, sunflowers, or other oily seeds. Using old coconuts that recently fell off the tree, harvest the milk, shred and press the meat against a piece of cloth to strain out every ounce of liquid, set the liquid aside to ferment for two days. The usable oil with float to the top. Scoop it off and add it to step 5 above. You may be stranded on a desert island, but you can still have skin as smooth as a baby.


Now for the hot water.

Hot Shower Ingredients:
A hole
Something dark (volcanic rocks, dark sand, black plastic sheeting, etc.)
Seawater
A pipe or a hose. You can use bamboo, hollowed out tree trunks or even fired clay, if you have plenty of time on your hands. If you're stranded, you do.
Fresh water.
A means to get the seawater to the hole and out of it, such as a pump or a hose.
Plastic sheeting, if you have it.
A bucket, even one with holes.

1. Dig a hole in the sunniest place you can find. It should be wide rather than deep.It should also be above the area you plan to shower and live.
2. The darker the bottom of your hole, the better. Cover the bottom with your dark material.
3. Fill the hole with seawater. If you can pump it in from the sea, that would be ideal. If not, well, start making trips with the bucket.
3. Wait.
What you're creating is called a solar pond. A funny thing happens when seawater is heated in a shallow pond. Here's a technical description, borrowed from Wikipedia. "When solar energy is absorbed in the water, its temperature increases, causing thermal expansion and reduced density. If the water were fresh, the low-density warm water would float to the surface, causing a convection current. The temperature gradient alone causes a density gradient that decreases with depth. However the salinity gradient forms a density gradient that increases with depth, and this counteracts the temperature gradient, thus preventing heat in the lower layers from moving upwards by convection and leaving the pond. This means that the temperature at the bottom of the pond will rise." In fact, it rises up to 100°C. At sea level, that's the boiliing point!
4. Siphon out or release the water from the pond when you need it. Note: this water is extremely salty, so it won't be much use to take a shower in it directly. Instead, you can direct it into a holding tank inside the tank that holds your fresh water. It's a bit of a big operation, but, hey, you've got time to figure it out.

There you have it. Soap and hot water!

A side note on other uses for your solar pond:
1. If you install piping into the walls or floors of your hut, you can run the hot water through it, keeping you snug and warm all winter without a fire!
2. If you have clear plastic sheeting, drape it over the salt pond at an angle, terminating in a bucket. Solar ponds evaporate, and that evaporated water can be collected and stored in the buckets. This is a lifesaver if there are no streams on your island!
3. Once the solar pond water evaporates, you have good, natural sea salt. You can use it to preserve your meats and fish. You can use it to build a Baghdad battery. YOu can even use it for various forms of agriculture. There's a reason salt used to be the gold of the world.

Of course, we all hope you'll never need this information. At least now you're better informed in how well Tuk's crew lives in Red Sand!