Soda ash is a chemical compound that is commonly known as washing soda because it is the main ingredient used in most powdered washing detergent and dishwasher detergent. Its chemical formula is NaCO(3), and its chemical name is sodium carbonate, which should not be confused with sodium bicarbonate, otherwise known as baking soda. The main purpose for soda ash is to raise pH, and it is used for this purpose in swimming pools, dyeing fabric and making glass.
Go to the grocery store. Be careful of the soda ash you are buying. Many that can be bought at a grocery store have unwanted additives in them. The "All About Hand Dyeing" website recommends that you buy Arm & Hammer's Super Washing Soda because it does not have any additives.
Go to a swimming pool supplies store. They will have pure forms of soda ash used to increase the pH of the water in swimming pools.
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If you don't have a swimming pool supplies store in your area, try a hardware store. They may also sell pool supplies.
Go online and buy from a supplier (see references below). Some will not require you to buy more than one pound of soda ash.
Check eBay or a similar online auction website. You might be able to get the soda ash for a cheaper price than at an online supplier.
Soda ash can have water molecules added to it, which makes it heavier. You will need to adjust how much you use depending on how much water has been added (double check package instructions). Keep completely dry forms of soda ash in an airtight container to prevent it from absorbing water molecules from the air.