Before something bad happens I'd like to get this clarified...

With my ratios worked out I figure my mixing "jar" will be sized @ 1.5L for a Noman extraction. I figured the best option would be a 1.5L wine bottle...
- figured glass "safer" than plastic
- tek specifies glass mixing container as well
- since NP would be extracted w/ glass baster, the long neck would help
- already have plenty of these lying around anyway

Then I've been reading a few threads here about how some ppl have had their glass jars or bottles burst, making a nasty mess...
What causes the pressure buildup? Is it shaking the lye & water or does it happen when you add the naphtha? Is this even a common thing? And what methods can be used to work around this safely?

I use mason jars, they work great.It is not an explosion. I believe its the lye etching the glass over time, so best to use 1 jar 1 time..they're cheap.

when mixing various things, reactions occur and it may heat up/build pressure. let it cool when this happens, so you can remove another variable that may add to you have lye on your face

It's not the heat it's the lye, as stated before.

E: Though technically heat helps the lye etch fissures into the glass.

PET plastic, the type with a 1 in a triangle printed on the bottom of the container is chemically resistant.
The 4L bottles of shellite I have used come in these bottles, great for extracting in and virtually indestructible.
Glass is nice but its not so easy to find something large enough for bulk extraction.

booldmt - I have used this method (wine bottle) without issues. The shape of the neck does indeed lend itself to efficient separation of liquids. Make sure that your glass baster will fit far enough into the neck. I would go for a plastic milk container though. They are cheap, strong and common. They can be squashed or cut as required. The only real issue is that they are normally translucent rather than transparent and it's therefore more difficult to see what's going on inside. The Milk bottle will say HDPE (usually on the base).

In my limited experience the pressure build-up is caused by heat and or shaking. It's a good idea to loosen the top occasionally with this in mind. HDPE plastic can of course be squashed a little before shaking/heating so that they will subsequently expand rather than burst. I believe it's the alkaline etching the glass rather than pressure build up that causes bursts though.

Plus, if it was an explosion you would have shattered Glass as well as lye on your face

Most pressure is built when acidifying or basifying solutions. If SWIM use sodium carbonate or sodium bicarbonate SWIM will make a lot of CO2 as it reacts. I recommend not completely capping your reaction container such that it can vent excess gas overnight.

Another common off gassing is from the lye based extractions when non-pure lye is used. If you use lye that contains aluminum (like most drain crystals) in a non-anhydrous environment it will react with the aluminum giving off hydrogen gas in large amounts.

2xAl(s) + 2xNaOH(aq) + 2xH2O(l) ----> 2xNaAlO2(aq) + 3xH2(g)

If SWIM sees metal flakes in their lye they are most likely aluminum, the drain cleaners use it to add pressure to their already powerful solution of clog dissolving and flushing. NaOH and KOH both perform excellently at dissolving organic material. (like hairballs/foodstuffs in the drain)

As long as SWIM doesn't cap their bottles while it is still reacting they should never have a problem.

SWIM can even use their experiment to fill balloons with hydrogen for the next day.

or

If SWIM wants to remove the aluminum from the lye easily (it is unfortunate that a magnet won't help) I recommend getting a plastic or metal screen and rubbing the lye solution against it. The NaOH or KOH will easily break into smaller particals which will fit through the screen, however the aluminum will not.

I recommend using SWIMS hands through PVC gloves for this procedure, and MAKE SURE THE GLOVES AND MATERIAL STAY ANHYDROUS. Works great.