The attempt to make a single solid mass of K-alum, using over 125 grams of McCormick K-alum powder, has resulted, in less than 2 hours, in:

enter image description here

So besides the growth around the loop of the seed "crystal", there are 3 other regions where crystals are forming.

Context: I am seeking a solid mass of K-alum to use as an astringent and antiseptic after shave (starting at 30 seconds into this video).

Questions: (1) Assuming that the over 125 grams of powder is large enough, will the bottom two or three masses combine into a big "cohesive/well-glued" mass? (2) Or should I dismantle the setup -- keep the seed-block, but heat the water to dissolve the other three masses to become very slightly under-saturated and put the seed back in?

Update: It has been under 3 hours, and the bottom three masses have combined into a single mass. I do not know how "cohesive/well-glued" that single mass will be when it is eventually removed and dried with a paper-towel.

The comment by @tstone-1 suggests this simpler approach for creating a single mass of K-alum: Heat about 200 ml of water to about 88C; saturate this with K-alum powder (about 100 grams); filter this solution into a paper coffee cup; cover cup and wait till most of the water evaporates; tear off paper cup to get solid mass. Question: how cohesive/well-glued will this mass be?

Update 2: Did end up with two consolidated masses, one from the starter near the rim and the other from the bottom three chunks; both are suitable for use after-shave as an astringent and antiseptic:

enter image description here

  • 1
    $\begingroup$ There is a good chance they combine if there is plenty material dissolved. If you are trying to grow a single crystal, you likely want to avoid that. I can't see it well enough whether your seed is actually good enough for a single crystal attempt. The question depends very much on your intention with the final crystal, but if you want to grow a crystal, you will want to use supersaturated solutions. $\endgroup$ Sep 8, 2020 at 18:10
  • 3
    $\begingroup$ Growing potassium alum single crystals is a typical experiment in our undergrad laboratories. From my very personal view and experience: While one can guess some larger single crystalline domains from your photograph, the stuff looks rather polycrystalline and aggregated. As you already noticed, this compound crystallizes very readily within hours (without seeding) and it is quite easy to obtain translucent and clear single crystals with edge lengths in the range of centimeters. Try to cool down the solution as slowly as possible, e.g. with the vessel placed within a larger bath. $\endgroup$
    – tstone-1
    Sep 8, 2020 at 18:13
  • $\begingroup$ Thanks -- have corrected my misuse of the term "crystal" and have made explicit the goal of the activity. $\endgroup$ Sep 8, 2020 at 18:45
  • $\begingroup$ Perhaps the proper question for me to ask is "How best to make a block of K-alum from powder?" with the post made above being my attempt at doing so. $\endgroup$ Sep 8, 2020 at 19:15

1 Answer 1


According to the Wikipedia article Potassium alum, the dodecahydrate melts at 92 to 95 °C. So if I wanted a large lump, I'd put the salt in a test tube (or some other container), dunk the container in boiling water to melt the salt, then pour the melted salt into some type of a mold to cool and solidify.

  • $\begingroup$ I tried the suggestion with a piece of Ammonium alum (which has a similar melting point as K-alum) that I had handy. On melting, the liquid was first colorless and then became light-green-blue in color. This liquid solidified as small clumps that adhered with each other -- but there is still some light-green-blue liquid left over. (So it is not like melting candle wax and re-solidifying it.) Is it known that melting K-alum and re-solidifying it will bring back K-alum? $\endgroup$ Sep 8, 2020 at 22:33
  • $\begingroup$ Not sure exactly how to explain that. // Another option would be to press the powder into a pellet. In the XRF lab I had dies that made 1/2 inch and 1.25 inch pellets. $\endgroup$
    – MaxW
    Sep 8, 2020 at 22:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.