You will hear me talk about using soft glass (soda lime glass) or boro (borosilicate) in my work, but what's the difference, how can you call glass soft (well of course when melted it is 'soft', but why is soda lime glass called soft and boro isn't)?  Now for the science bit folks.

All materials (and elements) have what is called a coefficient of expansion (COE), the temperature at which the material changes shape (in a nutshell).  In terms of glass it is often described by it's COE by a number, the higher the number the lower the temperature needed to change the shape of the molecules in the glass, ie melt it.  For example, the glass I use for my beads is commonly called 104 glass, this glass needs the lowest temperature of all glasses to make it start to melt which is why is it called 'soft'.  The COE for boro is around 33 so needs much hotter temperatures to work it and is the glass or choice for glass blowers, it also means it is great for sculptural work and because it is 'stiffer' doesn't tend to drip all over your workbench and also doesn't suffer from thermal shock as much, ie you can go back and reheat an area on a boro piece once it has cooled down a bit (still ouch to touch!), whereas if you did this with a soft glass bead it would go pop!. 

There are other glasses out there with COE's inbetween the boro and soda lime glass and can also be used for bead making, fused glass work and other glass work.  It is important never to mix glasses of different COE's becuase you can imagine if you have one glass that melts at a hotter temperature mixed with one that melts at a lower temperature and will cool at different temperatures as it cools there will be a bit of a fight going on and the glass cracks or shatters.  You can see a good example of thermal cracking of course if you put hot liquid in a cold glass (unless it is glass intended for this practice, drinking glasses will be made from differeent glass to the glass used to make glass tea or coffee cups).

Soda lime glass (soft glass, COE104) is so called because within it's chemical make up there is some lime in it and is the most commonly used glass for all sorts of things, not just beads, but also windows.  But here's the thing, you can't just assume that all soda lime glass has exactly the same COE so you couldn't smash up some windows and maybe a few glass bottles and make beads out of them all mixed up together as there are subtle differences in these glasses and you could find yourself with shattered beads!  If you do want to use recycled glass in your work it is best to not to mix them up together.  You can use VERY tiny amounts of glass with different COE's to decorate your beads (called frit) but never too much, otherwise you will have the shattered bead issue!  Actually I should say that it isn't just the COE of a glass that can affect incompatibility but there are other variables that can affect this.  I found this interesting which I found on a blog post by Lori Greenberg a US lampworker.

I hope that kind of makes sense, it is a BIG subject and there is quite a bit of chemisty and physics going on here.  There is more info on all this on the internet if you type in boro, soda lime glass, COE etc.  I have added a few links that are quite interesting if you would like to read more, but as I say it is a big subject.

From Warm Glass website

Something from Wikipedia (never quite like to use links from here, goes back to doing my degree where info from here wasn't considered to be good source data!).

What’s all this about glass being ‘soft’?