The best replacement for caustic soda is soda ash.

The best alternatives to caustic soda are rated as following:
★★★★★  soda ash
★★★★  lye
★★★★  lime
★★★  caustic potash
★★  sodium hydroxide
★  soda crystals
★  washing soda

Your alternative for caustic soda is not listed? Suggest!
Speaking about caustic soda, one probably shouldn't forget about making soaps.

Other search terms related to caustic soda and their ratings are:
★★★★★ making soaps
You have in mind a companion for caustic soda and it is not yet here? Propose!



Google Trends

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caustic soda
soda ash
lye