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Sorry for coming in here with little chemistry knowledge and a pretty inane question, but my googling skills have completely failed to cut through the enormous combined output of the baking brigade and all the lovely people making lovely things with this stuff. Unsurprisingly not one of whom has paused to mention what's actually happening on a chemical level.

For the uninitiated, salt dough is made by mixing half a part table salt with one part wheat flour, and one half to one part water, though recipes vary greatly. This is then moulded into the desired shape and baked in an oven at anywhere between 60 and 180 °C, resulting in a hard and surprisingly tough (to me anyway) finished article which is often used as a decoration on Christmas trees. But what is actually happening?

I can guess at something involving the re crystallization of the sodium chloride, but wheat flour, whilst being so incredibly common, is to me a completely incomprehensible mix of proteins, starches and all manner of other stuff. All of which involves chemistry way over my head.

I would greatly appreciate an explanation of what's going on in this hardening process.

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What you get is a polymeric (starch) composite, with salt as inorganic filler.

The salt particles give toughness to the resulting composite, while the starch phase, which is just the same as in your bread, minus the unnecessary blowing agent (baking powder, yeast, whatever), connects the particles in an elastic, flexible way. You get a material that is nearly as hard as a piece of rock salt, but much less brittle.

(Elastic here means more elastic than salt. Dry bread is quite flexible, compared to rock salt ;-) .)

See also https://en.wikipedia.org/wiki/Composite_material

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  • $\begingroup$ Thanks Karl, definitely helpful though your explanation is somewhat at odds with my (very limited) knowledge of how bread works. As I understand it, in bread the starch acts as the filler with some being turned to glucose to "feed" the yeast. The binder in bread is the glutens, which as well as being very long are able to bond with each other, forming the strong stretchy gluten web, which is somehow effected by the presence of salt ions in the water, though at much lower concentrations than here. $\endgroup$
    – user2630147
    Commented Jan 8, 2017 at 11:07
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    $\begingroup$ The gluten is afaik important during baking, when the leavening agent produces CO2 bubbles. These pop (coalesce) very quickly without he gluten network, which stabilises the dough. It's an effect called strain hardening. Expanding bubbles stretch molecules at their surface. That's why bakers like wheat: High gluten content. While the dough sets during baking, the previously partially dissolved starch macromolecules form a second, stiffer network. $\endgroup$
    – Karl
    Commented Jan 8, 2017 at 14:27
  • $\begingroup$ I guess for salt dough, it's not important, and rice flour (gluten-free) would also do nicely. $\endgroup$
    – Karl
    Commented Jan 8, 2017 at 14:31
  • $\begingroup$ Thanks again Karl, that's very interesting. The idea of gluten free salt dough is certainly amusing, perhaps the gluten intolerance "epidemic" will spread to Christmas trees... Could you perhaps edit your answer to elaborate on the formation of the starch network? Are we talking about starch gelatinisation? en.m.wikipedia.org/wiki/Starch_gelatinization $\endgroup$
    – user2630147
    Commented Jan 8, 2017 at 14:53
  • $\begingroup$ I'd hope the epidemic completely moves to the Christmas tree. Certanily wouldn't bother me there. ;-) This wp article is quite confusing, i'm not sure what they try to teach. I'd have to read up again on the starch network before expanding the answer. $\endgroup$
    – Karl
    Commented Jan 8, 2017 at 16:16

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