How do we call the empty space between sections of an architectural structure, or a building?
Joint, interstice, cleft, interval…
These are all synonyms, and in each different circumstance we will use the most appropriate term.
Now let me ask another question:
What do we put in the empty space I’m talking of?
Depending on the type of structure, we need to fill it with an appropriate material, that fulfils four requirements:
- joining sections
- giving appropriate elasticity and hardness
- allowing adequate expansion or shrinking of sections
- withstanding wear, for as long as possible.
You may want to think of bridges, of railway sections, of the wide pavements in an airport.
The engineers and architects who planned such structures, have created empty spaces between the sections of which each structure is made.
These spaces are «filled» with appropriate material, that performs the four functions I listed above.
What would happen if no such spaces were left?
Changes in temperature, load, and wear, at the points of greater stress, would generate cracks, erosion and damage, which could weaken the structure; sometimes dangerously so.
Architectural works in art mosaic are not free from stress.
The materials of which the artwork is made, as well as the structure surrounding it, have different degrees of expansion.
An artwork made of Venetian glass smalti has a reaction and expansion coefficient, that differs from that of the surrounding structure, which could be made of metal, or masonry.
Therefore, the artwork and the surrounding structure must be separated by an empty space, which lets the two parts (for instance, mosaic and wall) free to «move» according to their respective nature.
One of the best procedures consists of filling the empty space with an «expansion joint».
The usefulness of an «expansion joint», however, goes beyond letting sections free to move. Its further benefits include:
- closing the empty space, so that no water or other potentially deteriorating materials can seep in
- optically and aesthetically joining the artwork with the surrounding structure.
It is imperative, of course, that the appropriate material is chosen, having well studied and planned for its application during the project phase. Such material must be:
- chemically and physically compatible with all materials with which it comes in contact
- environmentally adequate
- aesthetically pleasant
- tested for long-term resistance over time
A professionally made mosaic artwork MUST include this structural detail.
Sticking tesserae on a wall or a floor is not enough to make one feel like a professional.
One must BE a professional.
The client who buys an art mosaic work must rely on this, without risking seeing his prized possession damaged on the next change of season.