The organs built by Cavaillé-Coll had a divided windchest for the foundation and combination voices. We have the advantage in knowing that César Franck was thorough in writing the registration cues, however one can be fooled without the understanding of the actual instrument. When Franck writes Anches, he requires not only reeds but the combination registers standing on a separate windchest including mutations and sometimes mixtures. On the other hand, we find not only foundation voices on the Récit. Usually Basson-Hautbois, by construction a reed voice, was standing on the main windchest together with foundation voices. In a similar manner, Voix céleste (undulating stop frequently used with Gamba) was on the main windchest for practical reasons. Cavaillé-Coll’s acoustic conception goal was a wide specter, layered homogeneous sound. The foundational 8’ voices were Principals, Flutes, and Strings. The 16’ voices formed a wide and mellow bass.
The mutations were meant to only support and intensify the main sound. Mixtures were meant to only enrich the plenum of the labial and reed voices and not to overtake them. The swell boxes were exceptionally effective and enclosed the Récit with a rich disposition, including 16’ voices and several reeds. The main steps in the layered dynamics, when performing the organ literature specific for this period, are reeds, couplers and swell boxes. The organist, even without changing one register, could begin his performance on Recit with a closed swell box, sooner or later switching to Positif and G.O. and step by step using couplers to add upper manuals. The next step would be adding Jeux de Combinaison from Récit to G.O., and in the end opening the swell shutters, in some cases adding 16’ voices. This way, very gradual changes of the dynamics can be achieved. At that time, the crescendo roller was not used and Cavaillé-Coll didn’t build it, just like fixed or free combinations (generals) on his organs. In relation to the effective swell box, it is interesting that Franck denotes full Récit as “pp” while the shutters are shut.
This may be the typical layered crescendo on a symphonic instrument:
- Pull all 16, 8 and 4 flue stops in all manuals except undulating stops. (The undulating stops in the Caen are the Voix céleste 8' and the Unda Maris. Undulating stops are considered effect stops and are never part of the tutti, since they weaken the sound volume). Couple all manuals. The French symphonic term for this is "Grands Fonds" - "all foundation stops". A crescendo can be accomplished by playing first on the Réc, then on the Pos, and finally on the G.O. Couple the pedal at first just to Réc, then to Pos (when you move to play on Pos), and finally also to G.O. (again when the hands start playing there). Obviously, another start of the crescendo may be required in some circumstances, for example the 8' stops only without any 4' and 16', which may be added at later stage.
- Now, if you draw the Anches 16, 8 and 4 on the Récit (you might also add the Cornet on the Réc) and close the swell box, you now have something which in French is called "Demi Grand Choeur" or "half tutti". Start by playing only on the closed Réc (coupling the Pedal only to the Rec.)
- In the same manner as before, start playing on the Positif. Couple the Pedal also to Pos.
- Next, start playing on the G.O. Couple the Pedal also to the G.O.
- Slowly open the swell box.
- Add the reeds to the Positif. (Possibly including any Cornet)
- Add the reeds on the G.O. (Possibly including any Cornet)
- Optionally, add mutations (= Alikvoten) and mixtures.
- Optionally, add Octaves Graves.
- You now have a tutti, or as the French term has it, a "Grand Choeur".
In fact, all this can be achieved by playing on the G.O. all the time, using the foot levers (couplers) to add different divisions at appropriate time. Since there is the G.O. unison coupler (enabling and disabling the Barker machine of the G.O.) you can sucessfully disable the G.O. stops if you need to do so.
Another note is necessary regarding the Prestant. Franck occasionally omits Prestant 4’ on G.O. and Positif (this omition is specifically required in the beginning of Finale) for the practical reasons: the swell box would not be effective enough for the powerful Prestant.
Finally, the beginning of the Chorale in A Minor is often mistakenly performed as an impressive toccata with plenum of powerful reeds and mixtures. However, Franck requires: "Jeux de Fonds et Jeux d´Anches de 8p. Claviers accouplées. Tirasse GO." That means only foundation 8’ and reeds 8' voices without any 16’, 4’ voices, mutations or mixtures. In this way, the chorale beginning gets more improvised, soft character. The final answer is given towards the end of the opus.
Tournemire, Widor or Guilmant were fully following this acoustic ideal. For Marcel Dupre, his ideal organ is the one from St. Sulpice, even though we can find a mixture plenum and solo mutations in his compositions. The term Anches now belongs only to the reeds. The diversion from Romantic sound is apparent on some organs built later in Cavaille-Coll’s career (St. Eustache) or reconstructions of his works (Ste. Trinite, St. Clotilde). There are more high-sounding mixtures, rich, colorful mutations and pungent reeds. The conceptual separation of mixtures and reeds is progressing. The culmination of the trend can be found in the original colorful registration by Olivier Messiaen. But even he follows the tradition of the Romantic sound using Recit with rich disposition or by plentiful use of reeds and swells.