DFHack
Utility to create sand/clay collection tiles
A feature which has been requested several times recently is the ability to create arbitrary Sand or Clay tiles in fortresses which do not otherwise have access to those materials.
Sand and Clay collection only work with SOIL tiles, so using the tiletypes plugin to try and place arbitrary materials does not work (since that creates MINERAL tiles), and ideally this should be possible without needing to convert the entire visible soil layer into sand/clay.
All geological regions (aside from a few biomes in particular) have four soil layers at the top, though in most cases only 1 or 2 of those layers are actually visible within the embark, the rest having been removed by erosion. One of these unused layers could be transformed into either Sand or Clay (of a particular type, selected by the player) and then an arbitrary tile's type and geolayer_index could be modified accordingly.
If we implement this as a Plugin (rather than as a Script), it would be feasible to scan the entire map for existing soil tiles in order to prevent any unintended material changes (e.g. if subterranean plant harvesting / tree cutting still results in the creation of Soil tiles).
Additionally, for embarks which occupy multiple geological regions, a larger number of unused soil layers would be available for use, permitting players to place sand, ordinary clay, and fire clay all at once; for embarks having only a single geological region, only two might be possible, so the ability to reuse already-modified layers might be needed in case players change their mind.
