Starting again after the first jar turned to soup
The first jar was a large coffee jar filled, in order of mistakes, with soil dug from the garden, a handful of moss pulled straight off the wall without rinsing, a small fern from a pot, and about a cup of tap water poured in because the soil looked dry. I sealed it and put it on the south-facing kitchen windowsill because that was where I would see it. Within a week the glass was running with water from morning to night. Within three weeks the fern had collapsed, the moss was a grey mat, and when I opened the lid the smell was that of a blocked drain. I tipped it onto the compost heap.
What went wrong, as far as I can reconstruct it. The garden soil was heavy clay and full of life that was not going to survive in a sealed jar, so it rotted. There was far too much water: a cup in a jar that size meant the substrate was saturated, and in direct sun the whole thing was a steam bath, with the inside temperature well above the room. The fern was probably cooked rather than drowned. And the south window meant the jar was fogged all day, so even if anything had survived it would have had no light.
The second jar, set up this morning, is a one-and-a-half-litre pickle jar on the north shelf in the hall, where the light is even and there is never direct sun. The substrate is a mix I made from coir, a little fine orchid bark and sharp sand, roughly three parts to one to one, over a thin layer of gravel and a sprinkle of charcoal from a bag of barbecue lumpwood, crushed with a hammer. The moss was rinsed in a bowl of rainwater and picked over with tweezers for anything that moved. Water went in by syringe: 25 millilitres of rainwater for about 400 millilitres of substrate, enough to make it damp but not shiny.
The rule I have given myself is that the glass should fog partly for part of the day and clear completely for part of it. If it is fogged all the time, there is too much water and I take the lid off for an evening. If it never fogs, I add 5 millilitres. We shall see in three weeks.