After the Portico of Glory, Cathedral of Santiago de Compostela (XII century)

Structurally, the organistrum of the Pórtico de la Gloria shares characteristics of instruments carved out from a single block of wood, and those built from pieces: the ribs are carved from a single block of wood, while the soundboard and the bottom are made of separate pieces, as well as the keyboard box.

The organistrum is a “bowed” stringed instrument, with a wooden wheel instead of a bow to produce the sound. The three produce a simultaneous sound, wich can be modulate by keys. It is closely related to the monochord.

Christian Rault, in his excellent study on the organistrum, mentioned above, examines four possible systems for the action of the keyboard, according to movement of the keys: rotation, sliding (like hurdy-gurdies), or balance.

It is crystal clear that the sliding system, used in almost every hury-gurdy, is a first order reference, and the typical structure of keys protruding through square hole on both sides of the keyboard box can

Two instruments of this kind can be found in Santiago de Compostela: The best known of the Portico of the Glory, the one we ca see in the photo above, and the less known but no less interesting present in the building next to the Cathedral, the Palace of Xelmirez.

The soundboard has several sound holes, apart from the magnificent rose with floral motifs, typical of Magister Mateo`s school.

The wheel is perfectly placed in relation to the bridge and the assumed string length.

It shows three thick strings.

There are two more examples in Galicia, both in porches of churches: the one in the Church of San Nicholas, Portomarin (Lugo) and the one in the Portico of Paradise of the Cathedral of Ourense

-As the strings are very long, its oscillation is so wide that they hit continuously the tangents of the keys, specially in high  notes. This breaks the vibration, producing noise and undesirable harmonics. This effect is particularly strong when the key approaches the string.

instrument, separating it from the knees of the performers. The person in charge of the wheel  should offset this effect by pressing the instrument tightly against the legs of both performers. This leads to a kind of struggle between the two musicians. The result is a tiring and unstable performance, with lot of noises.

The high prominence the organistrum in the Ordo Prophetarum that Actus stages guide us to make a new reflection about it, in order to make a confortable instrument. Finally, we have chosen the balance system as the best suited for the Portico instrument.

In the Portico of Glory we can see just one end of the keys, the top, the one handled by the performer with the fingers. the other end of the key is hidden inside the keyboard box. This arrangement invited us to create a keyboard of a type similar to the clavichord, that touches the string from below. The keys lie in repose beneath the strings, and this have no obstacle for its vibration, because there are no vertical elements obstructing it.

Among all the images of the organistrum found in Europe, the one present in the Portico de la Gloria has peculiarities that make it of special interest. We see the accuracy of the sculpture, the abundance of constructive details and the perfect proportions. Moreover, the keyboard, with its twelve keys, sets a conceptual challenge to us; and the unknown inner action, wich challenges us to give a constructive an functional answer to the enigma.