Cuocere / Degumming
To make the thread soft and white, the sericin needs to be removed by boiling the thread in a solution of soap and water. Alternatively, the thread can be used "raw" and the finished fabric de-gummed.
Exact details vary somewhat, but all involve near-boiling water and soap. I found the most information on sites for industrial silk processing such as FAO's Silk Reeling and Testing Manual.
Soap - Neutral soap made from pure olive oil or olive oil foots and soda.
Classic Ivory Bar Soap - Ph 10.4
Neutrogena Original Formula Facial Bar - Ph 8.7 - 9.2
Bon Savon has castile soaps with a neutral Ph, made from olive oil.
I've used "Kiss my Face" brand olive oil soap, and had good results. It's not neutral Ph, but it tests milder than Ivory (which I've also used with good results.)
Soda - Sodium carbonate (Na2CO3)
Sodium carbonate is also called soda ash and washing soda. The carbonate ions from dissolved sodium carbonate precipitate magnesium and calcium ions from hard water. These metal ions would otherwise combine with soap or detergent and form an insoluble scum that would stain the laundered clothes.
Na2CO3 = washing soda = sodium carbonate
NaHCO3 = baking soda = sodium hydrogen carbonate.
I called Arm and Hammer, and they said that their Super Washing Soda only contained sodium carbonate - no bleaches, perfumes or other ingredients, so that's what I use.
Water - Demineralized water. You don't want hard water minerals combining with the soap and forming scum.
I've used Brita filtered water and distilled water. I'm not sure how demineralized they are, but they're better than my (hard) tap water.
Degumming Formula and Directions:
It's okay if you need to use extra water. This formula was intended for industrial use, where everything used adds to the cost.
Expect to lose 20 - 25% of the weight of the silk in gum.