For starters I will say I hate static purity zones which mean that someone, who started game early or simply was lucky, gets some area with very good purity.
I like when in sandbox games you can progress by doing various tasks.
I love character progress system in Salem and HnH because you geat means - curios and inspirationals - to increase your skills in many different ways. Basically, everything you do in the game gives you those items.
Food is a bit similar - there are many different ways - hunting, fishing, farming, foraging - to obtain food and increase your stats. Although it seems that, due to bad balancing imo, it ends with using only few types of food anyway.
But purity seems to be just a grind, where you increase quality of products in one specific way. For farming it's repetitively replanting stuff. For ore it's searching for better mines with better rods. Now it seems we return to HnH's way of endless loop of increasing material quality then tools, then materials again and so on.
Inspirationals and food are two main types of items which let you progress, with equipment slowly becoming third one.
So I propose we bring another type of items - alchemy reagents.
Alchemy reagents
They could be used to influence alchemical composition of materials. Add some to fertilizer to boost trees or fields. Add some to a water filter / well to change water's alchemical composition. Burn some in fires when softening mine walls to affect ore purity, or add them to smelter fuel to affect metal purity. Add them to tanning tubs or clay throughs to affect leather and clay. Add them to coal piles, smokers, compost bins. Preserve the meat and change its alchemical values by smoking or curing it with the use of reagents. These are just some quick ideas, but the point is that alchemical reagents could be used to affect just any type of material.
Each reagent would have alchemical components, either just like most items now - 4 components which sum to 1. Or just the components which they would increase. In the first case, when applying reagent, final purity of an item would a weighted average of item's base quality with weight of 10, and reagent's with weight of 1. In the second case reagent's components could be added to item's/object's and then it all would be normalized again.
Now, where would the reagents come from? Everywhere! Just like inspirationals.
Some from mining and quarrying - typical minerals like salt, nitrates, ground gems, some oxides (I obviously have no idea about geology!)
Some from foraging and farming - plant extracts, fungi, weird stuff found in the wilds.
Some from hunting - animal extracts. Like dried and ground rabbit liver, squashed deer eye, ground bear tooth. Go wild
Some from fishing - fish / water plants extracts.
Some from processing materials as side results from smelting, carpentry, cooking etc.
And some from crafting - pure alchemy. Breaking down materials into alchemical ingredients.
Basically, there should be many different ways of obtaining those reagents. Some would be better (pure) and harder to get, and some would be weaker, but more common.
Using items to boost other items smells of tedious grind though. So, instead of using reagents on items themselves they should be used on tools required to obtain those materials. As mentioned before - wells, smelters, coal piles, fields, pots, tanning tubs, clay throughs, meat curing... thingies, ovens, smokers etc.
And then there would be diminishing returns for using multiple reagents in short time. For example, each use of a reagent on an object would reduce object's susceptibility to reagents by 50%, which would slowly increase back over time, say by 5% of (1-current susceptibility) per hour.
This way, it would take some time to reach good purity and there is again a possibility of speeding this up by using more reagents, but less efficiently.
Also, each use of an infused object would slightly pull its purity down, so that after reaching high purity you would have to occasionally use some reagent again to maintain the purity.
Metals
Another, related idea. What if we had only one type of ore, but the resulting metal would be determined by reagents we throw into smelter? Right now we use lime for iron. We could have some other reagents which would turn ore into copper / tin / silver / gold / lead. This way we could get rid of different ore deposits and instead of getting lucky and hitting gold vein we would have to make some effort and prepare some rare reagents to be able to smelt some of this precious metal out of our ordinary ore. Turning iron into gold, eh?