Can I use this for cold process soap making?

I believe so. This should be NaOH, sodium hydroxide. I know people use this to make soap but I don't know if they are making cold process soap........

Is it safe for Bathtub brass drain and pipe?

All the reviews I read on this said it was ok, and that was my experience. The shower drain had a metal cover. Since the product warns DO NOT bring into contact with aluminum, I removed the cover to the drain. I just didn't have a clue what the cover was made of. In the sink, I remove the stopper that sits inside the drain for both that reason, and I wanted to be able to pour the lye straight into the drain. I ran hot water down both for about 5 minutes. Then I used paper towels to soak and wipe up any water or dampness around the drain.I'm not sure I emphasized it enough, but the only surprise I got was when I opened the bottle of lye itself. When I removed the cap and started to tip it to put into the measuring cup, the tiny little beads of lye (the size of a pinhead) started to literally jump out of the bottle!! Just that very little bit that did this is what made me decide to pour it straight fro the bottle down the drain. Holding the measuring cup next to the bottle its pretty obvious that the bottle contains about 2 cups worth anyway, so I poured a cups worth (half the bottle) down the drain.The sink drain actually filled up to the top of the drain at the sink so I stopped pouring. The lye just sat there at the top of the drain. I didn't experience any volcanic outpouring from the drain, but since I read it was possible I decided to turn the measuring cup upside down over the drain just in case. Then I started to turn the water on ever so carefully. I had just the slightest amount of water dripping out of the faucet. I watched it run slowly to the drain and under the measuring cup. I didn't hear or see any crazy reaction, so I turned the water on ever so gently a little more. I slowly got to a point of a gentle stream of water and had it go long enough that I felt it would be about a cups worth of water. I then waited about 10 minutes maybe. (As you can tell, I didn't go by the directions exactly due to what I had to work with, but I certainly used them as a guide line, and the safety warnings as gospel). I then ran the water on hot for about 15 minutes, in which the whole time the water went down the drain perfectly and smoothly just as if it was a brand new drain and pipes!!I just used this yesterday and I'm still quite thrilled with how well it worked, especially after how long its been a problem drain, and how many hundreds of dollars we've spent over the years on store bought drain cleaners and local pluming specialists. I definitely recommend this stuff!! The only downside I see is that you MUST be VERY CAREFUL as it warns of it coming into contact with skin, eyes, aluminum, and of course water or any other liquid. Fumes don't seem to be a problem though as long as you don't have your face above the stuff when you're pouring it, or over the drain. I just left the fan on, the door open, and told everyone to stay out of the bathroom until I say otherwise.Sorry if I rambled on here. Its just that its been so bad for so long. I'm definitely ordering another couple of bottles so I can do my other drains. This stuff really isn't very expensive, especially if you're getting more than one bottle so the shipping isn't as much. Hope I helped you out a little. If you have any other questions feel free to ask. Take care, stay safe, and God bless, Nolan

Can this be used on a pop-up drain in a bathroom sink? How would you do that?

Probably only with DIFFICULTY. My plug wouldn't stay up, SO I had removed it completely LONG ago. I had unobstructed drain opening to work with. Suggest you phone HORN PLUMBING, my supplier, at 417-256-5191; ask them, staff knowledgeable and VERY HELPFUL.

How much of lye do I need to pour to unclog a completely stopped main line?

Not an easy question to answer with certainly. The blockage in a "Main" line can be caused by tree/plant roots, food and toilet waste and a combination of some or all. Also, where in the main the blockage is will determine how much Pure Lye to add. All these are probably unknown. If the blockage is caused by roots then Pure Lye will not be effective. We suggest putting in a full one pound bottle into the toilet and flush the toilet several times. You will have to wait a good hour before you may see some results. You may have to do this several time but even then there is no guarantee the blockage will be broken up due to the nature of the blockage and where the blockage is.