Potash is a broad term used to describe a variety of soluble compounds of alkali salts, mainly potassium carbonate, potassium chloride, sodium carbonate, and sodium chloride. These compounds are found in burned plant ashes as a portion of the leftover non-flammable material.
Potash acts as a flux for silica compounds. This is important and has been used as a glaze for pottery, which can melt and thus seal ceramic items without the ceramic item melting. Without this it would be very hard to waterproof earthenware, as earthenware tends not to vitrify at the temperatures present in firing, and so the earthenware remains porous.
It can also be used to lower the melting point of quartz which enables glass to be made at reasonable temperatures. Without this it is very hard to make glass without modern furnaces.
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Step 1: Source Ingredients
The main ingredient is wood ash which contains potash but also a lot of other substances such as Iron oxides, silicon oxides, aluminium oxide, calcium oxide, magnesium oxide, manganese oxide, phosphorus oxides and unburnt carbon.
Ideally you would use wood ash from a particular type of wood to ensure that each time you make potash the proportions of each compound are the same. Also some wood ashes have more potash than others so it is worth experimenting to find which wood has the highest yield of potash per gram of wood burnt. I will at some point add to this instructable comparing different woods and the potash yield. For this I just used ash that was in my fireplace and so probably contained some paper ash as well as wood ash.
The difference between potash and the other substances is the solubility in water. Potash will dissolve in water whereas the other compounds will not. This enables the potash to be separated by dissolving it in water and then filtering out the insoluble compounds.
Step 2: How to Make It
To make the potash, its solubility is useful as it can dissolve in water, whereas the insoluble other compounds will not. So by soaking the ash in water overnight the potash will all dissolve into the water. Then the solid waste can be filtered out. However, as I had no filter paper i instead left the ash mix and waited for the solids to sink to the bottom. I could then decant off the water and potash solution. This was not ideal as it left some small insoluble particles in the potash mix.
Once i had a rough solution of potash and water, I then just had to separate the potash from the water. The easiest way to do this is to evaporate the water and leave the potash as a crystal. To do this I placed the solution in a pan and placed it on the stove. To get the best crystals you would not boil off all the water and instead would stop heating once a few small crystals started forming and let the solution slowly evaporate and let large crystals form. However, it already had many impurities in it so I knew it would not make perfect crystals and so just boiled the pan dry.
Once the pan has cooled I scraped the crystals out. You could use the crystals in this form, but it is relatively impure potash and so might not work perfectly. To gain purer crystals you could redissolve them in water and filter again to get rid of the last insoluble compounds, and then recrystallise the potash properly.