Most Carolingian buildings, even the palaces of Charlemagne, were made of wood. Though not unknown, glass was rare. Brick-making was practically unknown. But lead plates and tiles were used for roofing and bells were made with some skill. Frescoes decorated the greater churches and monasteries.
The ninth and tenth centuries were in general a period of destruction rather than construction. New barbarian invasions and the civil and feudal wars were ruinous to all wooden buildings. The same held true for all churches built with wooden roofs. With the cessation of the invasions, the relative settling down of feudal chaos, and the growing monastic reforms associated with Cluny, forces were set in motion making possible the first real architectural revival in the west in the eleventh century.
To a French monk writing at this time the large amount of new building in stone made it seem "as though the world were throwing off its decrepitude to clothe itself anew in an array of white sanctuaries." It was to be a new age of stone. Building became an exciting passion. The amount of building increased as the century grew older. And when the new stimuli of reforms and crusades, and above all of the town boom were added, it went on even more rapidly in the early 12th century until it culminated in the Gothic furor.
A Norman abbot wrote in 1115:
"Who has ever seen or heard the like? Princes, powerful and wealthy men, men of noble birth, proud and beautiful women, bent their necks to the yoke of the carts which carried stones, wood, wine, corn, oil, lime, everything necessary for the church and the support of those working at it. One saw as many as a thousand people, men and women, attacked to the reins drawing a wagon so heavy was its burden and a profound silence reigned among the crowd pressing forward with difficulty in the emotion which filled their hearts."
It was, roughly speaking, during the century and a half following 1000 that the characteristic feature of what we call the Romanesque style were evolved. Yet it is difficult to be accurate in defining the particular features that distinguish Romanesque from earlier and later styles. It is hardly possible to say that the use of the round arch distinguishes it because it borrowed the round arch from the Romans, and there are pointed arches in Romanesque churches in southern France. There were many schools of Romanesque: the Lombard school, the Rhenish school, the Norman school, etc. Some unity was given to these various schools by traveling workmen and conquests.
Ordinarily the Romanesque church was inclined to be low and poorly lighted, to employ predominantly the round Roman arch, and to emphasize horizontal lines. It preserved and improved upon the modifications of the Carolingian basilican ground plan. The church became larger, and the approach to the altar from the nave more impressive. The original rectangular basilica became a Latin cross in form. The nave was of course for the congregation with its ground plan left open for the vista and for processions. The side or true aisles were the approaches; the transepts provided space for the officiating clergy and their assistants and for part of the communicants. In the feudal age the wings of the transept were reserved for the clergy and the aristocracy. Romanesque interiors were sometimes richly decorated with frescoes and mosaics under 'Byzantine influence. The facade was as yet undeveloped except for the sometimes extraordinary Norman western towers.
Better times, increased wealth, heightened religious emotion in general demanded bigger and better churches. The Romans had built three kinds of tone vaults, the domed, barrel, and groined vaults. During the early middle ages it appears that knowledge of how to build these tone vaults was lost in the west. But during the Romanesque period it was regained, and these three kinds of stone vaults began to be used.
The domed vault was least popular. To build a barrel vault over nave or side aisles requires an elaborate preliminary scaffolding for the whole area to be vaulted in order to center the arch of the vault until the mortar or cement hardens. The vault itself must be of considerable weight to spring successfully over any considerable area. To support it, therefore, the walls upon which it rests must be of corresponding wight, and it is dangerous to pierce them with anything but relatively small windows for fear they will collapse. Moreover, every stone vault contains a force called a thrust, a tendency to bulge outward, which, if the walls are not strong enough to contain it as well as hold the weight of the vault, causes them to collapse. The thrust of the barrel vault is strongest at the haunches of the arch and is, of course, continuous for its whole length.
In the groined vault, however, the continuous thrust of the barrel vault is broken into individual thrusts which are concentrated along the lines of the intersections or groins of the vault, and brought down to a definite point at the four corners of the vault. In the groined vault, therefore, there are two points of thrust, the haunch and springing point of the arch, but they are isolated along the lines of intersection of the two barrel vaults which go to make up the groined vault.
If some external support to the walls of the nave or side aisles could be applied at those points where, in a groined vault, the thrust is concentrated, then the function of the walls would be limited to supporting the weight of the vault, rather than both supporting the weight and containing the thrust. They could accordingly be made lighter, and could less dangerously be opened up with windows.
Thus, from the point of view of a well-lighted interior, groined vaults were superior to barrel vaults. All this knowledge was learned by the Romanesque architects only after costly and sometimes disastrous experiments with actual buildings.
Stone vaults were first of all thrown over the narrower spaces of the side aisles. Skill and knowledge had to be acquired before the builders dared put them over the wider areas of nave and transepts. The building of a groined vault necessitates the division of the area to be vaulted into squares called bays, for obviously the intersection of two barrel vaults of equal height can only cover a square.
For both barrel vaulting and groined vaulting, builders found it advantageous to mark the limits of the bay by spanning the area to be vaulted with arches of masonry, called ribs, that could be used as partial supports of the vaults themselves. The framework of transverse, diagonal, and longitudinal ribs formed a kind of permanent centering and acted as a partial support for the weight of the vault carrying and concentrating the thrusts of the vault to one specific spot on the nave wall. Each particular rib is carried to the floor as a distinct part of a clustered pier.
But the problem of light was not yet settled, because the builders did not dare to raise the very heavy vault of the nave so high as to admit a clearstory. Light had to come in, therefore, only from the windows of the well-developed triforium gallery and of the side aisles. Without a clearstory the concentrated thrust of the nave vaulting could be met by the vaults of the triforium gallery, which carried them to the tong pilaster buttresses built on the outside walls of the side aisles and triforium, at those points where the thrusts of their own vaults were concentrated by ribbing. So, by the use of buttresses, piers, and ribbing the problem of supporting the stone vault had been solved. But this still did not bring in enough light.
Norman architects solved that problem. They not only introduced an extra transverse rib between the diagonal ribs of the nave vault, but they raised the vaulting of the nave high enough above the triforium gallery to permit a clearstory. so, the Normans had taken another step in preparing for Gothic by combining ribbed vaulting with clearstory and elementary flying buttresses.
The chief new structural addition made by Gothic architects was to bring the primitive flying buttress of the Romanesque from under cover of the lean-to roof of the side aisles, and to raise it to meet the higher thrust of a higher nave vault and carry it to the ground.
Thus what the Gothic Cathedral was from a structural point of view was skeletal framework of side-aisle wall buttresses, flying buttresses, clustered piers, and ribbing in perfect equilibrium, built to support the weight of and counterbalance and carry to the ground the thrusts of the stone vaults of the side aisles and the nave.
With such a framework, walls as such served no purpose except
to keep out the light. Gothic architects then proceeded to remove
all the flat wall surfaces and to substitute for them windows
of colored glass. Hence the walls of side aisles tended to become
mere glass interrupted only by the outside wall buttresses. The
clearstory became in effect one continuous sheet of colored glass
interrupted only by ribs running down from vault to pier or to
the floor. When a gabled roof was substituted for a lean-to roof
over the side aisles, the triforium could be transformed into
glass. In total effect, therefore, the Gothic church is a soaring
glass house held together by a skeletal framework of stone, or
a vast "vaulted glass cage."