Pros and Cons:
Making Your Own Lye

Yes, you can make lye from scratch.

Lye making can be a bit of pain, depending on your living conditions. If you've got the space, ashes from your fireplace that would get thrown out anyway, and a burning curiousity to understand what soap-making was like in the days before lye could be delivered to your front door at the click of a button... then, by all means, give it a try.

If (like me!) you're in a tiny apartment with no place to set up a lye making system, no convenient fireplace to plunder ashes from, and little patience for imprecision (such as not knowing the exact concentration of your lye-water)... then you'll probably want to sit this one out.

  Commerical Lye Homemade Lye
Pros: - Purity
Lye marketed for soap-making has a high level of purity. Avoid draincleaners, even if labeled "100% lye" (they usually aren't!)

- Precise Concentration
You control the concentration of lye. This allows you to use the easier cold-process method.
- Knowledge
Learn how lye was made thousands of years ago.

- Self-Sufficiency
Do it yourself, and don't worry about where to find lye.

- Use Up Waste Products
If you have hardwood ash on hand (from a fireplace), you can get some use from it, rather than just throwing it out.
Cons: - Cost
You must purchase the lye, and possibly pay additional costs for shipping a hazardous material.

- Time
If lye is not locally available, you will need to wait for it to be shipped to you.
- Raw Materials
Lye may be easier to find then the materials, if you don't have access to ash and straw.

- Unknown Concentration
You can not use this lye for cold-process soap-making (you will need to use hot-process).

- Time Consuming
You'll need to spend several days letting your lye water build up it's potency before it can be used.

- Softer Soap
Lye making produces potassium hydroxide, NOT sodium hydroxide. This creates a softer (or liquid) soap.

- Safety
You'll need to take proper safety precautions while making lye.

Overall, making your own lye introduces more work to every step of the soap-making process: making lye is more time-consuming than purchasing lye; homemade lye will force you to use hot-process methods, which are more intense than cold-process, and you need to add steps and ingredients to harden your soap at the end.

Which is why I stick with commercial lye.

However, if you really want to know how it's done - for the sake of historically re-enactments or extreme self-sufficiency, here's a basic set of directions to get your started: How to Make Lye.