Bending and drawing glass tubing is a handy skill for managing laboratory glassware. Here is how to do it.
Note About Glass
There are two main types of glass using in a lab: flint glass and borosilicate glass. Borosilicate glass may carry a label (e.g., Pyrex). Flint glass typically is not labeled. You can bend and draw flint glass using just about any flame. Borosilicate glass, on the other hand, requires higher heat in order to soften so that you can manipulate it. If you have flint glass, try to use an alcohol burner, since too high of heat may cause your glass to melt too quickly to work it. If you have borosilicate glass, you’ll need a gas flame in order to work the glass. The glass won’t bend or else will be very hard to bend in an alcohol flame.
Bending Glass Tubing
- Hold the tubing horizontally in the hottest part of the flame. This is the blue part of a gas flame or just above the top of the inner cone of an alcohol flame. Your goal is to heat the section of glass you want to bend, plus about a centimeter on either side of this point. A flame spreader is helpful for a gas flame, but not absolutely necessary.
- Rotate the tubing to make certain it is heated evenly.
- As you heat and rotate the tubing, apply gentle and continuous pressure where you want it bent. Once you feel the glass start to yield, release the pressure.
- Heat the tubing a few seconds longer. It starts to bend under its own weight, you have overheated it!
- Remove the tubing from the heat and allow it to cool a couple of seconds.
- In a single motion, bend the slightly cooled glass to the desired angle. Hold it in that position until it hardens.
- Set the glass on a heat-resistant surface to allow it to completely cool. Do not set it on a cold, un-insulated surface, such as a stone lab bench, since this likely will cause it to crack or break! An oven mitt or hot pad works great.
Drawing Glass Tubing
- Heat the tubing as if you were going to bend it. Place the section of glass to be drawn in the hottest part of the flame and rotate the glass to heat it evenly.
- Once the glass becomes pliable, remove it from heat and pull the two ends straight away from each other until the tubing reaches the desired thickness. One ‘trick’ to avoid getting a bow or curve in the glass is to let gravity help you out. Hold the glass tubing vertical to draw it, either pulling up on it or else letting gravity pull it down for you.
- Allow the tubing to cool, then cut it and fire polish the sharp edges.
Among other uses, this is a handy technique for making your own pipettes, especially if you find the ones you have on hand are either too big or too small to deliver the desired volume.
Here are some causes and fixes for common problems:
- Glass Won’t Get Soft – This happens if the flame temperature is too low to heat the glass. The solution is to use a hotter fuel, such as gas.
- Glass Gets Too Soft, Too Fast – This is caused by using too high of heat. Back off on the length of time you put the glass in the heat, hold it further from the hottest part of the flame, or use a fuel source that burns with a cooler flame.
- Glass Has Bumps or Crimps – This can happen by bending the glass more than once or by letting it get too soft so that its weight starts to pull it down. The solution to this problem is experience and practice since there is a certain amount of ‘art’ to knowing when to remove the glass from the flame to bend it or pull it. Just know that once you decide to bend/pull, it’s a one-time deal. If it doesn’t work, it’s unlikely you can reheat the glass and get a better outcome.
- Glass Tubing Seals – If the inside of the tube seals, it’s because the glass got too hot. If you are bending the glass, remove it from the heat sooner. If you are pulling glass, let it cool a bit more before drawing it. Note that you may wish to purposely seal the glass. If you do, just heat the tubing in the flame, rotating it, until it seals shut.