Les roches ignées
Les roches ignées se forment dans les zones volcaniques par solidification des masses en fusion. Ce sont les plus anciennes, les plus durables et les plus résistantes des pierres. Le granite est la forme la plus connue de roche ignée, mais il existe également d'autres types de roches granitiques comme la pierre de Logonna.
Le granite offre une large gamme de couleurs et une densité qui permet un beau poli. Les principales sources de granites sont la Scandinavie, l'Afrique du Sud, le Brésil, l'Amérique du Nord ou la Grande Bretagne.
Les roches sédimentaires
Composées de particules d'autres roches comprimées au fil du temps, elles sont plus poreuses et plus faciles à travailler que les roches ignées, elles tendent à donner des surfaces à texture.
Les types principaux de roche sédimentaire sont le grès, le calcaire et le gypse, tous trois composés de particules qui se sont agglomérées sous l'action d'adhésifs naturels.
Le grès est formé en grande partie de particules de silice, matière cristalline qui le rend très durable, mais il peut être fragile si l'agent de liaison entre les particules est faible. Les couleurs sont en général terreuses : rouge, ocre ou gris. Son aspect poreux fait qu'il ne se prête pas au poli et il faut prendre garde à bien protéger ses voies respiratoires car la poussière de grès est très nocive.
Le calcaire se forme par la déposition de matière crayeuse, telle que des fragments de coquilles ou les squelettes minuscules d'organismes marins. Il contient d'ailleurs parfois des coquilles et des fossiles identifiables. Malgré leur homogénéité, de nombreux calcaires présentent des strates qui définissent le grain de la pierre et entrainent desfissures horizontales, ce qui limite la dimension des sculptures en calcaire.
Le calcaire est généralement facile à couper et à travailler tout en présentant une bonne résistance aux intempéries, ce qui en fait un matériau largement utilisée pour la taille de pierre.
The gypsum stone
The gypsum stone, also called gypsite, is a common salt rock of the sedimentary basins. The gypseous stone is white or yellowish and often crossed by grey or yellow veins constituted by silica or other elements. The gypsum stone is used to industrially make the plaster by submitting it to the heat.
Is is a soft rock easy to extract and to cut. The hardest varieties, allowing a beautiful polish, are called alabaster (gypseous alabaster) and used in sculpture.
There are many sedimentary deposits. The most furnished countries are: Germany, Chile, the United States, Great Britain, Italy in particular in Tuscany and in Sicily, Mexico, Poland, Russia. In France, there are numerous exploited, exploitable or known sites, besides the mining veins in Paris Basin, Massif Central, Vosges, Hautes-Alpes, Jura or the Aquitain basin.
Les roches métamorphiques
Sous l'effet de la chaleur, de la pression ou de certains phénomènes chimiques, les roches ignées et sédimentaires subissent une modification de leur structure qui les transforme en roches métamorphiques. Les plus connues sont le marbre, l'albâtre, l'ardoise et la stéatite.
It is a metamorphic stone, derived from limestone and existing in a wide range of colors.
It is the noblest stone, the archetypal stone of the sculpture. In Antiquity all the stones used for sculpture were called marble. The marble is easily recognizable in its glittering aspect once polished and being remaining cold under the hand.
It is a rather hard stone to be worked with tools and a hammer then has to be worked with carborundum to prepare the polishing phase. It produces sharp bits when hitted that might cut, so be aware to protect your eyes and even your face because those bits might jump rather violently.
During the Antiquity, the quarries of Pentélique, which overhangs Athens, of Paros, Proconnèse in the Sea of Marmara or from Aliki to Thasos are the most important. In the roman time the famous marble quarries of Carrare, in Tuscany, begin to be exploited. It is also at that time that begins the extraction in the quarries of Alentejo in the center of Portugal. Discovered at the same period in Wallonia, the famous Belgian black marble and royal red. In France, we extract the marble in Pyrenees; the North of France counts some quarries, for stones or aggregates. There are also some quarries in the Alps, in Italy and in France: green marble or serpentines, in particular in the Aosta Valley and in the valley of Ubaye.
The most famous marble for the sculpture is the Carrara marble, famous for its whiteness and its purity (it contains very few veins). It is still nowadays the most active marble extraction site in the world.
There are two types of alabaster stones: limestone type alabaster and gypseous type alabaster.
The limestone alabaster is also called former Marble onyx or Egyptian alabaster or alabaster of the Bible or oriental alabaster, because the first objects we knew in alabaster come from Far East. It is a calcite hard enough to mark the white marble. It almost always presents on its surface yellow undulations more or less dark, sometimes even dark red. It is extremely rare to find perfectly white limestone alabaster. When hitted, it breaks in a crystalline and streaked way, what gives it a semi-transparency, because the light has a much easier access in the thickness of this stone than in a marble for example which has a composition of an infinity of small strips which break rays of light without letting them enter. When polished it looks like marble.
The gypseous alabaster or former alabastrites, is a variety of gypsum, compact copper sulphate lime. It can be scratched with the nail. It is usually of a milky white color and its break is grainy and often mat. This variety of gypsum with fine grain is extracted in particular in English quarries or in Tuscany. The alabaster is used as decorative stone. Its tenderness allows to sculpt it in elaborate forms. Furthermore, considering its solubility in the water, it must be used inside. It loses its transparency, its shine and its solidity when exposed to the fire, because it then changes into plaster (sulfate of calcium or sulfate of lime).
The alabaster is a magnificent, rather soft stone, often translucent with veins and "clouds" which can be various colors. It is however necessary to be very careful because the translucent alabaster goes pale under the shocks, so be careful when using tools if you want to obtain a beautiful translucent polish at the end! Certain alabasters are translucent, others are white opaque and some are smoked (opaque with black trails), each of these types is of a great beauty and takes splendidly the polite. In a general way, alabasters white and orange are soft, while others blue or brown can be very hard (they are called alabaster-onyx).
The soapstone is a very soft rock, mainly consisted of talc.
Because of its ease of cut, it was abundantly used on sculpture, mainly for seals. Nowadays, it is also used on the manufacturing of stoves and fireplaces, thanks to its capacity to store the heat or in model making.
It is an easy-to-work stone because it is very soft and can be very well polished, giving beautiful results. It also presents the interest of presenting a very large range of colors and sometimes surprising combinations, what can cause happy surprises, but can also oblige to revise a little your project during the work to adapt it to these colors. The only way to have a precise idea of the colors of a soapstone is to polish it, it is thus interesting to make a stage of polishing during the work to see the aspect of the stone before finalizing the shape of the sculpture.
Soapstones often present the inconvenience (direct consequence of their tenderness) to be fragile and very often with weaknesses, it is thus necessary to work them very carefully. It is important to identify quickly the weaknesses, generally visible or recognizable by the tone when knocked with a tool. Then it is necessary to decide whether to blow up the weakness (if it does not disturb the project, it is the safest solution because then the stone will be healthy) or to integrate this one into the creation by being very careful not to break the stone. Then be careful to always work this part in the direction of the weakness to avoid to open it and with a big delicacy. It is often preferable to take the initiative before it breaks by using some transparent liquid glue in the weakness to reinforce it, the glue will spread an solidify the stone, but you will still have to remain careful and you will have to repeat the operation when getting into the stone.
If broken, there is a special glue called Stéacolle which allows to repair a break, but it will be useful only if the break is not too important, if your sculpture broke in two, it might be preferable then an awkward repair to realize two smaller sculptures with the broken pieces... But it is your decision!
Last point, be careful during the polishing of the stone, a large quantity of water insinuating into the weaknesses of the stone turns it to be much more fragile, then take care to manipulate the sculpture with precautions... Most of the breaks that happened to me occurred during the polishing... Then do not hesitate to put some glue in the weaknesses before beginning this stage.
Where to find stones for sculpture?
Art material stores
Several solutions to find stones for sculpture: there are art material which sell stones by kilo (rather expensive) as Boesner (the store of Champigny proposes a lot of stones, mainly alabasters, marble and soapstones of any sizes and colors), Géant des beaux arts (mainly small-sized soapstones) or Rougier et Plé. The problem with these stores is the price by kilo of stones which is rather high and the lack of diversity... Furthermore when a stony load is delivered, the nicest blocks are the first to be sold (do not hesitate to subscribe to the Boesner newsletter which warns when a load of stone has been delivered), it is thus necessary to know how to choose, especially as sometimes very hard stones can be mixed with soapstones, so hard that they can't be cut.
An other solution is to go directly to quarries. The access to quarries which are not in service anymore is normally forbidden, but in those which are still in activity, the persons in charge are often very kind and keep small blocks for the sculptors (they are rubbishes according to their criteria of form etc., but for a sculptor it's perfect!) and help you even sometimes to transport stones up to your car, all this for a very moderate price, or even often free of charge... Do not forget to thank them warmly and to propose them a banknote to thank them, they will be even nicer for the next sculptors who will visit them!
There are guides and websites on which you can find quarries everywhere in France, but it is necessary to be able to distinguish sculptables stones from others (which can always be used as bases!).
How to choose a stone?
From my point of view the choice of a stone is a very personal approach based mainly on a subjective feeling of attraction for a certain shape or a certain color. There are however some elements which can be checked to make sure of the quality of the stone.
In most of the stores selling stones, you will find, or can ask for wet sponges which will allow you to clean the stone to reveal its color and possibly identify weaknesses. Make sure that the stone is not crispy (if some water has infiltrated in it) and that it is not crossed by too large weaknesses. If you're looking for a translucent alabaster, you can check its transparency by creating a shade on its surface with your hand, you will then slightly perceive its transparency. To verify the depth of a weaknesses you can (delicately!) give small knocks on the stone and listen to the sound of it: if it sounds matt it is often sign of big weaknesses.