Why does the fabric need drying when using Aluminium acetate concentrate?

A student asked me yesterday why the fabric needs to be dried between mordant application and dunging when using Aluminium Acetate Concentrate, because other recipes go straight to the wheat bran/chalk bath. I can't find any scientific explanations anywhere unfortunately, but I will give it a go and try to explain how I understand it:

Keep in mind, that all Aluminium acetate versions can only be used successfully if a tannin is used on the fabric beforehand. Aluminium acetate in any form (unless as concentrate) will not be sufficient as a stand alone mordant for cellulose. 

Working in Aluminium acetate Concentrate

Aluminium acetate concentrate on the other hand, due to its concentrated form and strong presence of acid, will impregnate the fabric through print or 'massaged' application. The strong presence of acid will penetrate and settle within and on the fibre; just like tannin, the strong acid is used to attach or in this case 'transport' the Alum to the heart of the fibre. Once delivered, we have to remove the 'vehicle', in this case the strong acid. That can only be done by drying the cloth fully - hence sniffing the fibre to make sure that no 'vinegar' smell is still present. 

If that step is omitted, the alum will still be hitching a ride on the vehicle (the strong acid) and will only be washed away when added to the dunging solution.

Adding Chalk to water for dunging 

Therefore any recipes stating to simply mordant cellulose in an Aluminium Acetate solution, even when followed by dunging, which is already one more step in a right direction than most will give you, will not be as effective and long lasting. 

 And I know I am repeating myself here, all Aluminium acetate versions can only be used successfully if a tannin is used on the fabric beforehand. Aluminium acetate in any form (unless as concentrate) will not be sufficient as a stand alone mordant for cellulose. 

Dunging mordanted linen

Even when dunging is applied, the Alum acetate would only be on the surface and washed away to some extend.

Last but not least, what is dunging?

Excerpt from 'Using Alum in your studio' Professional Dyers Course:

 Because we are omitting the tannin in this recipe, we have to use another 'fixer' to fix the mordant into place. Traditionally, cow dung was used to neutralize and fix mordants on cellulose.. The active ingredients are enzymes and phosphates, that will not only fix the mordant, but also remove gum thickeners from printed fabrics by adding wheat  bran to the dunging mixture. Luckily, we have an alternative these days and do not need to go out into the field to collect cow dung. Chalk, Calcium carbonate, is now used to neutralize and fix mordants into place. 

I hope that will explain it a bit. I am not a chemist, unfortunately and my understanding of chemical reactions etc. is limited. I can only pass on what I have learned through research in conjunction with my experiences and therefore the way I am understanding it. :)