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I am getting ready to try steam distillation following this guide which references an earlier chapter which says "If using grease as recommended by your instructor, lightly grease all joints." Since i am just doing this on my own when is grease recommended in a general case, and what would my specific case (orange peels with possible use in candy) change those general recommendations.

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The greasing is not needed for the specific cases of steam distillation of volatile essential oils, perhaps should be even avoided.

Using light greasing(*) of glass joints (not limited to distillation) is recommended:

  • When there is expected significant pressure difference, compared to ambient pressure, like vacuum distilations.
  • When there are present harmful substances that could escape via loosen joints.
  • When the assembly is being manipulated, mainly before or after use, grease somewhat suppresses its tendency to accidentally disassemble. (not a reason to use it, rather a side effect)
  • When eventual presence of traces of grease does not affect the quality of product.
  • When there is danger or experience of joints getting stuck together.

Avoiding greasing is recommended:

  • When there is not expected significant pressure difference, compared to ambient pressure.
  • When eventual presence of traces of grease could affect the quality of product.

(*) - just enough to be able to wet both surfaces of the joint by mutual rotation, with visual control. Better repetitive adding small amounts until enough than using too much.

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    $\begingroup$ +1 "When there is expected significant pressure difference," that's a bit woolly. Should be "when you are going to run the glass apparatus at a lower than ambient pressure, i.e. with a vacuum regulator." Because it's extremely difficult to make it air-tight without grease. And if it's "mechanically challenged": fix that, or don't assemble it at all. ;) $\endgroup$
    – Karl
    Jan 18 at 20:33
  • $\begingroup$ As I say - significant pressure difference. // You have misinterpreted that. There is nothing to fix. $\endgroup$
    – Poutnik
    Jan 18 at 20:51
  • $\begingroup$ But what do you mean by "mechanically challenged"? $\endgroup$
    – Karl
    Jan 18 at 21:01
  • $\begingroup$ @Karl It might not be the best fitting expression. I as in the update, I mean there are scenarios, not limitied to distillation, that involve occasional manipulation and grease little bit helpts to hold it together. // I remember my high school class graduation lab work, with distillation of ammonia for later titration in 1980s. The lab assistent was unhappy due large amount of broken glass. $\endgroup$
    – Poutnik
    Jan 19 at 10:56
  • $\begingroup$ OK, I see what you mean. That works, although in my own (painful) experience, the flask with your product of three days of work then still falls off when you least expect it. Perhaps you had better grease than we did in the late 2000s. ;) $\endgroup$
    – Karl
    Jan 19 at 21:28

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