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I prefer to use Fels Naptha for my soap, and I've been using it for years with great success. I've also used Castile soap, but I didn't like it as much. In fact, I used it to wash my floors and clean my house to use it up because it was that bad for laundry. If you don't live on a farm or have kids that spill every other bite on their clothes, it may work for you.
Homemade Powdered Laundry Detergent Ingredients
How to Make Powdered Laundry Detergent
1. Unwrap the Fels Naptha and place it on a microwave safe plate. Microwave the soap until it's completely puffed up. This took 2 minutes in my microwave. I suggest microwaving at 30 second intervals until it looks like this. Do not touch the soap! It will be very hot. Open the microwave door and let it sit for 20-30 minutes.
2. Once the fluffy bar of soap is cooled off. start breaking it up in to smaller pieces over a bowl. I just crumble as much of it as I can by hand. It will crumble very easily.
3. There will be some chunks of soap left. You can use a food processor to grind it up, or use a masher like I do. I've used a potato masher and this round tool that has sharp edges. I don't know what it's for, but it works very well on soap.
4. Once it's all a powder, you should have about 1 cup of powdered soap. Add 1 cup of borax, 1 cup washing soda, and 1 cup oxygen powder if desired. I've made it both ways, and I like it with the oxy clean better. You can definitely leave it out or use less if you want. Give it a good stir to get everything mixed up.
This is what my finished laundry soap looks like. You can see there are still some bigger pieces of soap, I've never had a problem with it in my washer.
I use 1 tablespoon per extra large load in my non-HE top load. I've heard that this works well in an HE washer because it is low suds, but always read your manual before you use it.
After you make this is a great time to clean your microwave. The soap will make your microwave smell like soap, so I leave the door open to air out and then clean it. If it still smells, leave the door open overnight. I've never had the smell linger longer than overnight with the door open.