In this instructable I'm going to show you how to cut a bottle with a soldering iron. I also made a video, which you can watch here: https://youtu.be/JF6M9a_bJdo
This method does take quite a bit longer than some of the others. This bottle took me about half an hour to cut. What I like about using this method is that it's been a lot more successful for me, and gives me almost complete control of where the bottle cracks. Sometimes, though, it will crack wrong, but that rarely happens.
A couple things to remember before starting: only use the tip of the soldering iron and not the thicker part. Also make sure there isn't any solder on the tip. These are some of the things I've tried that caused the bottle to crack wrong. I'm assuming it's because the heat wasn't as focused as you want it to be with this technique.
What you will need:
Step 1: Score the Bottle
We'll start with scoring the bottle with a bottle cutter. Remember to get a good score line, and only do one pass. Going over the same score line multiple times will make the cut more difficult. While making the score line you can tell when it starts to overlap when the sound changes to a crunching noise.
This score isn't ideal. There is a section that got missed because the bottle isn't perfectly round, and part of the score line strays off. But with this method of cutting, these flaws are not much of an issue.
Step 2: Start the First Pass
Choose a place to start that is away from the flaws. With a hot soldering iron, place the tip against the score line and hold it there for about 5 seconds; the exact time is not too critical. Then move the tip a bit and do the same. Continue doing this all the way around the bottle. This won't be enough to cut the bottle, but a lot of the time it will start a crack along the score line.
Step 3: Dealing With Scoring Flaws
If you happen to have stray score lines, like I did this time, just follow the line where you want the bottle cut. Also, if you have any places that didn't score, just follow the path where you want it to cut. Alhough I didn't do it this time, it helps to draw a line for the path with a marker, so that you have a visual reference to follow.
Step 4: After the First Pass
When you finish going all the way around, examine the score line and see if there are any cracks started. Sometimes they start with the first pass, and sometimes they don't. The crack will be shallow, and possibly hard to see, so try to get it to reflect. Check from above the score line, and below. If you see any cracks started, goto that spot and continue from there.
Signs to look for a starting crack vary: It could look like an obvious crack, or could look like the score line getting slightly thicker, or it could look like a reflection or glare on the inside of the glass. This time it started by looking like a reflection.
Step 5: How to Start a Crack (if One Hasn't Started Yet)
If you don't have any started, just pick a spot near your starting point. Before continuing, you'll need to get a crack started, which I'll explain how to do now.
Similar to the first pass, place the tip against the score line. This time hold it for longer, about 8-10 seconds for each position. Also, don't go all the way around the bottle, maybe about an inch or so. Sometimes you'll hear the bottle crack and sometimes you won't so check again for any signs of a crack starting then go back to where you started, and start again. Repeat this over and over, until you see signs of a crack starting.
Step 6: Growing/Guiding the Crack
If you can see the glare of the crack while you're heating it with the soldering iron, watch for it to grow. Sometimes it will fade while the crack is still starting, but that's fine. Once it does start to grow, you won't need to hold the soldering iron in the same place as long, so move it away from the crack as it grows. At this point, the crack will follow the soldering iron. If you do happen to have any places where there is no score line, keep following the path you want the crack to go and it will follow the soldering iron.
Step 7: Finishing the Crack and Separating the Bottle
When you get close to the start of the crack, sometimes you will see the two ends meet, but sometimes it will seem like they don't want to meet. If at this point the crack stops growing, the cut may be complete. Try separating the top of the bottle, but don't force it apart. If the cut is complete, it will separate easily. But if it feels like it doesn't want to separate, try heating the crack a bit more. Usually just heating the last bit helps, but sometimes you need to heat random parts of the crack. Just remember to not force it to separate, because that could cause it to crack randomly.
Step 8: What Next?
Now that the bottle is successfully cut, it's ready to be sanded and polished. That needs to be done because right now it's basically a broken piece of glass, and can easily cut somebody. You can go here to see my instructable for that: Polishing-Bottle-After-Cutting
To learn how I made my jig, check out my instructable for that: Make-a-Bottle-Cutting-Jig-Teardown
If you want to see the video version of this instructable, you can watch it here: https://youtu.be/JF6M9a_bJdo