Pierre apparente (visible stonework) is a French mason's technique of pointing coursed rubble masonry (moellons) to give refurbished new-looking wall with replaced lime pointing. It stands in contrast to a fully rendered rubble wall (with enduit or crépi).
The Abbé HP has always been puzzled how the artisans of masonry can achieve such a clean and pretty result, with the rubble's stone faces emerging from a sea of pointing, that might in one's own hands have looked little more than a schmooey mess.
Stage 1 is to use a masons pick to take out all the old lime pointing to a depth of maybe 5 cms. Then one sees if the rubble stonework can ever actually look pretty! Any stonework repairs required are carried out at this point.
Stage 2. On a big job one uses a lime-mortar 'concrete pump' - more or less like a motorised toothpaste tube for elephants - to squirt lime-based mortar (lime chaux + sharp sand) into the exposed joints. Lots will fall to the ground and have to be re-used or cleared up. Then the mortar is troweled hard into the joints by hand while it is still quite runny. Lime mortar, unlike Portland-cement-based mortar, does not adhere very well unless pushed hard home.
Then the magic wait - that only experience knows - that allows the mortar to 'go off' enough to work on.
Stage 3. Surplus mortar is removed to exposed the rubble faces with a stiff wire brush, and each stone's face is brushed up as clean as possible. The lime mortar falls away from the face in a way that only lime mortar allows. Careful artisans can achieve a really beautiful result, and this restoration process can be repeated many times over hundreds of years, as with the 18th century school canteen example in the pictures.
Stage 4 cleans up all the fallen mortar and the rest of the building site. Lime mortar blends with the rough rubble stonework of tuffeau field-stone that is so typical of Touraine to make a really charmng blond wall that catches the sunlight so well.
In the 17 and 18th centuries pierre apparente was only used for utility buildings, being definitely third class after cut stone pierre taillée and its cheaper cousin, full lime rendering crépi. The château of Richelieu was obviously in cut stone, the hôtels particuiers in render, and humble buildings like this one in pierre apparente. Likewise, the roofs of quality houses had steep slate ardoises Touraine roofs, while this one has a Poitevin flatter-pitch roof with humble Roman tiles tuiles rondes.