In Reality most Soils contain Clay to some Extend.
And Humanity has Learned fairly fast how to Extract Clay from Normal Soil when they were missing a Natural Clay Reserve close by.
Especially for Pottery this is not uncommon either.
To Produce Clay in Reality you would usually Mix Soil with Water in a pot and then let it Sit so you can afterwards take the Clay off the Top as it will be bound to the Water while the Soil will mostly gather at the Ground.
Repeating this Process a few Times you will then get a Mix of Water and Clay.
This Mix is then Dried by putting it into a Linen (or other) and letting it Dry for several Days.
This should be fairly easy to do in the Game. To begin with Processes in the Game are Simplified so we can Recreate the rough edge of the True Process.
Implement a New Device "Clay Tub"
Add a 2 new Abilitys to the Material Preparation Skill at lvl 90 "Extract Clay" and then "use Clay Tub"
Extracting Clay from Soil will be done by putting 120 Fertile Soil 5 Linen Cloth and 30 Water into the Clay Tub.
The process will take 3 Ingame Days and Produce 60 Clay
This would Allow Players to Create Clay from Soil and thus Fix the Shortage of Clay on the Map.
It would be Fairly Hard to do this on Large Scale. So Clay Deposits would still be highly Valuable.
But it would make sure that the Game will not Run out of Clay completely. And it will also Reduce the Strain put on the Natural Clay Pits thus Prolonging their Life Span.