I tried shikakai, amla, rice rinse water and combination of these.

Now I am pretty much settled with this recipe.

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300 ml of rice rinse water
2 teaspoons (10ml) of shikakai powder
1 teaspoon (5ml) of amla powder

Add shikakai and amla to rice rinse water. Bring to a boil. Simmer 5 minutes over low heat. Let it cool. Strain with tea strainer. Use half of it per hair wash. I pour the liquid in a Rubbermaid dressing bottle with small tip. Pour over my scalp and rub. Put on a shower cap. Let it sit for 5-10 minutes. Rinse.
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Rise rinse water is white translucent water you get when I rinse regular California rice before cooking. There is saponin in rice rinse water. When I shake rinse water, it bubbles up like very diluted shampoo.

When I used shikakai alone, or rice rinse water alone, it did not clean my oily scalp well. It was still oily. So I combined the two. Amla adds body to my hair. But if I add too much amla, my hair does not feel silky. So I just add half the amount of Shikakai.

Also I feel a significant difference when I boil and simmer shikakai soup (?). I wonder if someone else here has similar experience.

When I just added powder in hot water, the liquid was somewhat drying for me. So I needed to pre-oil my hair. But if I boil, shikakai liquid not only cleans but conditions my hair, making it easier to comb. My hair feels like I used regular conditioner. Smooth. Also it is not so drying. I don't need to pre-oil.

I seem to be able to extract something from powder when I bring to a boil and simmer.

Saponin is found in variety of grains, such as wheat, soybeans, red beans and chickpeas. But I have not tried any of them yet.