Calvi noticed the remoteness of the young painter from the "old style", that means from the Mannerist painting of Procaccini, Fontana and Samacchini who often, "wanting to correct nature, they deform it and deturpate it" and as in his early work, Guercino was "all based on the natural, from which he religiously copied everything in its very crudeness and simplicity. He took the very high lumen to achieve the effect of a great stain that he knew sweetly tune, and his things seemed painted in chiaroscuro rather than not; but the use to portray the continuous truth took gradually mastered such and such color daring, almost another Caravaggio gave to all in the eye, and was deservedly to meet with universal approval."
The Lanzi finally sees the youthful manner of Guercino as "full of strong shadows with very sharp lights, less studied in the faces and in the butt, pulling the yellowish meat, and in the rest vague of colorful; way that remotely resembles Caravaggio: it does not even resemble Cento, but Bologna still has some sage in San Guglielmo."
Biographers rightly approach Guercino more or less closely to Caravaggio for his reference to reality, but Guercino should not be associated to Caravaggio for the use of light contrasts: in Caravaggio they are a means to emphasize the plasticity of form, while in Guercino the luministic search is an end in itself, that aims to achieve a purely light effects. The large spot, the chiaroscuro, "protest this antiplastic intent in favor of an outdoor coloristic exaltation, against Caravaggio’s intentions, could not have succeeded more antithetical": the luminosity of Guercino "is never stretched to the revelation of a reality that in Merisi assumes intonations most raw and dramatic."
In 1612 the Canon Antonio Mirandola, the future president of the local monastery of the Holy Spirit, came from Bologna to Cento. He was an art lover, who encouraged the young painter by introducing him to the charcoal drawings of the painter Pietro Faccini (1562-1602), a pupil of the Carracci, who will bear in mind in its projects, and above all giving him the first commissions: that was how the Guercino painted (ca 1612) Two angels with the shroud and the San Carlo Borromeo to the Santa Maria Addolorata church’s frescoes, the Eternal Father and the Annunciation for the Church Holy Spirit (1613), the three blades of the parish of Renazzo di Cento (1614-1616), including the Madonna with the saints Pancras and Chiara, the Madonna enthroned with saints Francis, Anthony abbot and Bovo and The miracle of St. Charles Borromeo. Scannelli and Calvi noted about the latter "with the study our Giovan Francesco followed the footsteps of Ludovico Carracci." It was also noted as artificial lighting and the family atmosphere of the painting has been influenced by an altarpiece by Lavinia Fontana in 1590, the Nativity of the Virgin, then in the Bolognese church of San Biagio, which Guercino could know well.
In the light and shade of the house Provencal (1614) "increasingly could be seen how the Barbieri had studied over the works of Lodovico, because these terms are made in imitation of those famous of the Carracci in the Favi home in Bologna", but a discussion deserve the landscapes of the Pannini house (1615-1617) where the Guercino found a very personal item: no school references, free from constraints of academia, his freedom of representation leads him to manifest his love for nature and for the life of the fields, the pleasure of observing everyday scenes with freshness and simplicity, as shown by The Harvest, A Road of Cento, Boys playing in front of the church of San Biagio in Cento, The Rough, The Landscape with Linen in the Sun; a taste that is more constructed, also in the paintings of the "wonderful" Landscape in Moonlight with carriage of Stockholm or the "exquisite" Landscape with swimmers women Rotterdam. That should also be smoother result witnesses the Passeri, where he writes that in the scenic representations were "of great benefit contiguous Rusticana campaigns and sites of his native land, where he lived for a long time."
For the Mirandola canonical he painted in 1615 Four Evangelists, now in the Gallery of Dresden, three of which, taken from Mirandola in Bologna, were noticed by Archbishop Alessandro Ludovisi - a few years later, on February 9, 1621, he will become Pope with the name of Gregory XV - who wanted to know the author and bought them, behind advice of Ludovico Carracci, for the tidy sum of twenty-five crowns each. Ludovico Carracci was not limited to this: on October 25, 1617 the painter wrote to Don Ferrante Carli of Parma praising Guercino as "the great draftsman and colourist: he is a monster for nature and miracle to astonish those who see his works. I do not say anything: the first painters look stupid in comparison to him."
The Guercino was reputed as master and took the initiative in 1617 to found a school of painting in Cento: his friend Bartolomeo Fabbri gave him two rooms and they agreed "from Bologna, from Ferrara, from Modena, from Rimini, from Reggio and from France many young people [...] twenty-three pupils he had, no one could be said to be less loved than the others [...] and his guests in Cento were the Marquis Enzio and Cornelio Bentivoglio". That was his Academy of the nude, known at the time as long as he lived in Cento, where Guercino drew generally "the naked with charcoal, slightly tinted paper, and so great and so easy he did it, with a spot in which the smiting reflection of light, stood out the main obscure, and with few steadfast plaster lamps or white lead."
In 1618 Guercino accompanied in Venice, thenks to the suggestion of Mirandola and equipped with a collection of his drawings, a canonical of Cento, Pederzani Peter Martyr, who showed them to the well-known artist Jacopo Palma the Younger, to have an opinion on possibility of this young painter eager to improve in his art: it is said that the Venetian painter expressed his admiration by saying that "this beginner knows much more than me", and then accompanied him to Venice showing the paintings of the best artists.
The knowledge of the last Venetian paintings - by Titian and Jacopo Bassano in particular - confirmed him in his vocation to the chromatic sensitivity, which put in place in 1620 in Clothing of St. William of Aquitaine, painted for the church of Bologna San Gregorio and now in the Pinacoteca di Bologna, where "everything has a great and masterly character, the colors may not be better and what is called spotting taste is brought to the highest degree; the lights shine in the midst of the freshness of dough and a few major well-leased dark increase the painting a force and a relief that enchants. "The Marangoni considers it his masterpiece for "its warm atmosphere like scattered in a golden and luminous dust that bathes and drowns things, making airy denser shadows with a single result that rare and showing us Guercino as one of the greatest and most original of luministic renewal masters."
Feature of this and later works is the construction of the composition whose vital lines form a rhombus in which encloses the core of representation - in St. William the vertices of the lozenge lie in the Virgin’s head, the bishop of monaco and in the knee of the saint - a choice that gives vivacity to the composition, in opposition to the "firm coldness of the canvas rectangle."
So it is in the contemporary Saint Francis of Assisi in Ecstasy with St. Benedict and an angel, painted for the church of San Pietro in Cento, and now in the Louvre, where the diamond composition also adds the spiral movement of the body of Francis, "as that of a snake that is fascinated "by the caster angel music. This "great painting", repeatedly replicated by Guercino, was admired in the eighteenth century: Giuseppe Maria Crespi included it in the background of his portrait, preserved in the Wadsworth Museum in Hartford.