In response to Thud's question re my post of 25th April, as to whether these kind of stone/rubble walls were always rendered (and most Czech buildings are indeed rendered), I asked a friendly local builder who specialises in historic buildings to enlighten us as to why. In brief, this is what he said:
Apart from the obvious climatic reasons (hard winters, hot Summers) the rendering serves as both an aesthetic and a strenghtening layer. The oldest ordinary houses (as opposed to forts and castles etc) were made of locally collected stones from fields etc, which were packed, together with mud/clay into rows of wooden shuttering - similar to the ones used now for concrete. The shuttering would be removed after the layer has hardened, and moved up so another layer could be formed, and so on. Given the width of these walls (over a meter thick) the packing method produced durable walls that lasted for centuries. People with more means, or with easy access to quarried or found stone that chipped into more or less regular blocks would use these in ways similar to the English dry-stone walling, in that the blocks would form the outer skins of the wall, which would be then filled with loose stones - except here, too, clay would be used. Very thin layers between the blocks, and as filler for the rubble in between. Both these sorts of walls would then be painted with thin clay render, leaving the stone structure quite visible, only in later centuries thicker render took over as covering, using lime as well.
It was fascinating to listen to this knowledgeable man who was clearly aching to tell me all he knew. We decided that we'll find time to go and photograph the houses in the area with the view of writing about different techniques for the blog. Or start another blog altogether.
Meanwhile, shuttering came off at the Forest house from the new concrete 'collar' with armature which will hold the old walls from pulling apart, and help to hold the roof structure. Hmm: concrete... doesn't sit so well with the old techniques in principle. Feels like a nasty tooth filling. But it seems necessary, as the house was so full of cracks and the walls were bulging so badly. We'll see. Maybe better to fill the tooth than to have it pulled?