I'm convinced that most mold issues come from exposure at collection, but you can make the problem worse by creating good conditions for mold growth. I find bleach solutions do as much harm as good. Chestnuts are high in carbs and very attractive to mold. It is a balancing act trying to create good conditions for cold stratification and germination without encouraging mold.
The first step I take is to wash them individually under water.
Cold stratification requires both cold and moisture. For chestnuts it is 60 to 90 days. With sufficient moisture they can germinate closer to the 60 day mark. As moisture levels drop, cold stratification slows. If moisture levels drop low enough, cold stratification stops and they just go into sort of a suspended animation. Of course, moisture is one of the conditions that encourages mold. So, it is a balance.
The next step I take is to hydrate the nuts. I separate them into groups as I wash them. Mold moves quickly from nut to nut, so if you do everything in one batch, you have more chance of losing everything. I put each batch in a clean pot with water to soak over night and fully hydrate.
I like to use long-fiber sphagnum because it has some natural antifungal properties. The best is live sphagnum but that is hard to find most places. Home depot caries dried long-fiber sphagnum and it second best. I soak it in water so it absorbs all it can. I then put each group of nuts in a ziplock bag. I take a handful of sphagnum and squeeze it as hard as I can squeezing all the water out of it I can. This is about the right level of moisture. I put one fist full in with each bag. I label the bag, zip it half way closed, fold it in half, and put the bags in the crisper.
From here, there are a couple methods. For the first 60 days, you can ignore them as far as germination goes. The only thing you are checking for is mold. If you see signs of mold, try to keep the mold nuts away from all other nuts. Either discard moldy nuts or wash them again and put them in their own bag. Wash all the other nuts without mold in the bag again under running water. There should me no need to rehydrate. Discard the bag and sphagnum that was in the bag. Put the nuts in a new bag with new sphagnum. With any luck you won't have mold issues. It will largely depend on the source for your nuts.
After 60 days, a new factor comes into play. When a chestnut germinates, the radicle emerges from the pointed end and uses gravity to determine which way is up. This determination actually occurs a short time before the radicle is produced. The top growth will come from the same point and use light to determine which way is up. So, when you plant a nut, you want to plant it with the same orientation it had at germination. If a root radicle starts growing thinking one direction is down and you change the orientation, it will change the direction of growth, but it takes time and you can get a kink in the root that can be a problem in the future. A soft bend is not a problem but a sharp kink will be.
There are several strategies to deal with this. One is to simply plant the nuts after they have had at least 60 days of cold stratification but before root radicles begin. Plant them with the point to one side. If you are planting in an 18 indoors, put the chestnut in the corner with the point in the middle of the 18. Some folks put their nuts on top of the medium in an 18. I prefer to press mine down and cover them with media. This seems to reduce mold issues for me. The downside with this method is lower germination rates, and you are taking up nuts that will never germinate. One year Wayne and I compared our methods. I think there is a thread on here. He cold stratified for 80 days and got over 90% germination. I had plenty of nuts and 18s and was more concerned with timing than germination rates.
Another strategy is to orient the nuts so they are sitting on their flat side and let them germinate in cold storage. With this method, you check the nuts every few days after 60 days and plant them as soon as you see root radicles. It can be a pain with ziplock bags, so some use other kinds of plastic containers for this. After 90 days, just plant any nuts left.
I'm happy to answer any other questions you have as you go. I brought over some good threads from the old forum that are good background reading.
Thanks,
Jack