[Catalist] Possible effect of Saliva when cutting Glass Tubing?

[Michael McGarry]

Greetings Leon,

Thanks for your prompt reply to my 02052016 post to CATALIST.

From your supplied URL:

"The amount of energy required to cleave the silicon-oxygen bond decreases by a factor of twenty in the presence of water. This indicates that the water molecule fits into the crack tip and converts a silicon-oxygen bond into two silanol (SiOH) groups."

"Small diameter rod and tubing can be broken easily by hand. The glass is first scored with a file. If at all possible, the scratch should be made with one stroke of the file because sawing with the file tends to widen the scratch and lowers the chances for an even break. The glass should be supported by the bench top because considerable pressure (F/A) (about 3 to 6 pounds [force]) is needed to make the scratch. Saliva or water is placed on the scratch, which will lower the strength of the glass by about 20%. The rod or tubing is then grasped firmly with the scratch between and opposite the thumbs. The glass is bent at the same time it is pulled apart and a clean break should result."

Do we agree that 80 % of the energy needed to snap the scratched glass tubing is supplied by the mechanical stress of simultaneously bending and pulling apart the glass tubing at the scratch, with 20% of the energy supplied by the cleaving of the silicon-oxygen bonds along the scratch in the glass by the polar water molecule in the saliva. I am uncertain of the role played by OH - ions in the saliva. Further research is required. Most importantly we know from experience that one must have the correct proportion of bending and pulling to produce a ‘clean’ break in the scratched glass tubing.

Thanks and Best Regards,

Mike McGarry

    
> On 2 May 2016, at 6:34 AM, Leon Harris <leon at quoll.com> wrote:
> 
> "Saliva or water is placed on the scratch, which will lower the strength of the glass by about 20%."
> say  https://chemistry.boisestate.edu/richardbanks/glassblowing/glassblowing_history.htm <https://chemistry.boisestate.edu/richardbanks/glassblowing/glassblowing_history.htm>
> 
> So the optional bit probably relates to the argument "is 20% worth the bother"?
> 
> But yes, hydroxide in saliva, little that it is, is supposed to enhance the break.
> 
> Also note that there is non-negligible OH- in neutral water.
> 
> In my experience of breaking tube, it seems to work best if you get it within a few moments of scoring: after that the benefit falls off.
> 
> More on this later, when time permits....
> 
> Cheers,
> Leon
> 

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