The history of St. Isaac's Church dates back to 1706, when the first temple was built for the Admiralty Shipyards. The first building was located on the Admiralty Meadow opposite the Admiralty gates.

Here, on February 19 (March 1), 1712, Peter I and Catherine Alexeyevna were married.

However, just a few years later, the construction of a new church began. The need for its construction arose because the first church no longer met the demands of the rapidly developing Saint Petersburg. The proximity of the second St. Isaac's Church to the Neva River was damaging the building. Thus, the third St. Isaac's Church appeared. It was consecrated in 1802, but the new temple soon required reconstruction. It is from this moment that the history of St. Isaac's Cathedral begins, which residents and visitors of the Northern capital can see today.

In 1809, a competition was announced for the construction of a new temple. A mandatory condition was the preservation of the three consecrated altars of the existing cathedral. The competition program, approved by Alexander I, was composed by the president of the Academy of Arts, Alexander Stroganov. It stated: "To find a means to decorate the temple... without covering... its rich marble attire... to find a dome shape capable of giving grandeur and beauty to such a famous building... to devise a way to decorate the square belonging to this temple, bringing its circumference into proper regularity."

Architects Andreyan Zakharov, Andrey Voronikhin, Vasily Stasov, Giacomo Quarenghi, Charles Cameron, and others participated in the competition. But all projects were rejected by Alexander I because the authors proposed not a reconstruction of the cathedral but the construction of a new one. In 1813, under the same conditions, another competition was announced, and again none of the projects satisfied the emperor. Then, in 1816, Alexander I entrusted the engineer Agustín de Betancourt, who had arrived from Spain and was chairman of the newly formed "Committee on Construction and Hydraulic Works," with preparing a reconstruction project for St. Isaac's Cathedral. Betancourt proposed to entrust the project to the young architect Auguste Montferrand, who had recently arrived from France to Russia. To demonstrate his skill, Montferrand made 24 drawings of buildings in various architectural styles (though technically unsubstantiated), which Betancourt presented to Alexander I. The emperor liked the drawings, and soon a decree was signed appointing Montferrand as "Imperial Architect." At the same time, he was tasked with preparing the reconstruction project of St. Isaac's Cathedral with the condition to preserve the altar part of the existing cathedral.

In 1818, following Alexander I's directive, Montferrand developed a project that provided for the preservation of most of Rinaldi's cathedral (the altar part and the sub-dome pylons). The bell tower, altar projections, and the western wall of Rinaldi's cathedral were to be dismantled, while the sub-dome supporting pylons of the southern and northern walls were preserved. Columned porticos were planned to be erected on the north and south sides. The cathedral was to be crowned with one large dome and four smaller ones at the corners. The height of the vaults remained the same, which complicated the project development, and the overall composition of the building was disproportionate: the monumental portico and the large central dome, supplemented by the small corner domes, "pressed down" the portico. The project of the cathedral in this form was approved by the emperor in 1818. In the 1820 publication of his project, Montferrand included an interior image of the cathedral that gave a misleading impression of its internal perspective: the drum with window niches would not have been visible from the viewer's position.

As noted in his monograph by Nikitin, Montferrand took as models the buildings of the Paris Pantheon (dome with colonnade, portico, interior treatment) and the Hôtel des Invalides (dome covering construction). Montferrand's project (in terms of architectural composition, but not decoration) was not an original work but a compilation, which was common among many, even the most prominent architects of that time.

Striving to preserve Rinaldi's cathedral, Montferrand provided for an increase in the building's size only in the east-west direction, thus making the plan rectangular with a 4 to 7 ratio. Four new pylons increased the building's width by the width of the transverse nave. Taking into account the new porticos, the plan formed an almost equal-armed cross. Two old pylons, reinforced on the western side, and two new ones became the support for the dome. Rinaldi designed the dome with a diameter equal to the side of the square base; Montferrand placed a new dome on the same base, whose diameter equaled the diagonal of the square. Thus, the drum of the new dome hung over the vaults of the side naves. Together with the columns surrounding the drum, the diameter of the new dome was two-thirds larger than that of Rinaldi's dome.

The construction management was entrusted to a special commission. Its chairman was Count Golovin, a member of the State Council; members included Minister of Internal Affairs Kozodavlev, Minister of Spiritual Affairs and Public Education Prince Golitsyn, and engineer Betancourt.

The organization of work and all economic aspects of construction were entrusted to the commission (according to the commission's regulations approved only in July 1820), while the management of the actual construction work, resolution of all technical issues, and supervision of the architect's actions were assigned to Betancourt. Quality control of materials was entrusted to the architect. A special commissioner was appointed for their acceptance and storage. Betancourt, busy with work on other projects and often absent from Petersburg, limited himself to attending commission meetings and resolving issues related to the building's foundations.

On June 26, 1819, the solemn laying of the new cathedral took place.

In 1820, Montferrand published an album with 21 engraved plates. To his two unchanged plans, he added a longitudinal section, a general plan, Rinaldi's church project, as well as wall painting projects, two perspective views, and an interior image. The project again attracted specialists' attention. Architect A. Modoui, a former member of the "Committee on Construction and Hydraulic Works," sharply criticized it. On October 20, 1820, he submitted a memorandum to the Academy of Arts with remarks on the 1818 project. The memorandum was accompanied by drawings demonstrating Montferrand's errors.

Modoui petitioned to stop all construction work except stonecutting and dismantling parts of the structure intended for demolition. On June 14, 1821, at the Academy of Arts meeting, the creation of a special committee to review Modoui's remarks was announced. At the committee's first meeting (August 1821), two letters from Modoui (to Golitsyn about stopping work and to Olenin about issues raised in the project remarks requiring priority consideration), his memorandum, and Montferrand's rebuttals were heard. It was decided to review the approved plans, facades, and sections of the building and inspect the building itself in Montferrand's presence. The first inspection took place on August 15.

The essence of Modoui's remarks, which caused a great resonance, boiled down to three main points: doubts about the foundation's strength, the danger of uneven building settlement, and incorrect dome design, whose size exceeded permissible limits and risked collapse due to resting on pylons built at different times.

According to Modoui, Montferrand lacked the proper qualifications and experience for such a serious project and, starting it, did not understand the difficulties involved. Furthermore, Modoui noted that when beginning the building's reconstruction, Montferrand had no clear understanding of the existing foundations, did not know how deep the new ones should be laid, and planned to make them from rubble stone slabs—a method unsuitable for such a complex structure. According to Montferrand's response, he inspected the foundations under the building's northern corner and, together with a commission member, examined the cellars. Regarding the driving of pile foundations under both wings of the newly constructed porticos, which Modoui considered excessive and causing "unnecessary large expenses," the committee deemed the architect's actions correct.

Modoui believed that the four pylons supporting the lantern with the dome of the third cathedral were "almost impossible" to keep, as the foundation under the "auxiliary supports" would not withstand the load of the existing pylons. Two old pylons and two newly constructed ones, according to Montferrand's plan, were to support the new lantern and dome. Modoui pointed out that pylons built at different times would settle unevenly. The committee concluded that preserving the old foundations under the existing pylons made erecting a heavy stone dome unsafe due to uneven settlement.

Learning of the committee's conclusions, Alexander I instructed it to correct the project while maintaining the condition of "preserving, if possible, the existing walls, and moreover, the old and new foundations." It was also prescribed to keep the main features of Montferrand's project—five domes and columned porticos. The committee was given discretion over the interior space design, the main dome, and the building's lighting. Montferrand was allowed to participate in the work on general grounds.

Following the competition, the Committee could not make any decision: the projects were submitted to Alexander I for consideration. The latter probably realized he had set an unsolvable task for the architects and gave no orders. The Committee's work was paused until February 1824, when a decree was issued to continue design work. It again repeated the emperor's conditions; a concession was made to allow changing the elongated plan (proposed by Montferrand) to a square one, giving architects some freedom but requiring dismantling part of the already built foundation. Dismantling the eastern pylons was also permitted, which helped increase the sub-dome space and create a structurally correct connection between the drum and the dome. Thus began the second stage of the competition to correct the project, in which Montferrand himself participated. He studied the competition projects and revised his own, adopting some solutions from the projects of Mikhailov II and Stasov, as well as proposing his own ideas to correct previous errors. The revised project was submitted to the emperor on March 9, 1825, and approved on April 8 of the same year.

The new, corrected, and supplemented Montferrand project was again recognized as the best and approved by Alexander I. The construction commission was reorganized, including members of the abolished committee reviewing Modoui's remarks. Past errors were taken into account: construction no longer proceeded on a broad front, working drawings were developed (which had not been done before), and calendar plans and estimates (approximate, for one year, with the total construction cost still undefined) were prepared. The new project enlarged the main dome, and four small bell towers marked the corners of the central square. The slightly elongated basilica plan with three naves was skillfully masked by four symmetrical porticos. As a result, the temple acquired a more traditional five-domed appearance typical for Orthodoxy, but the external symmetry came at the cost of losing the altar apse projection in the eastern part. Montferrand took into account the advice and remarks of the leading Russian architects, engineers, sculptors, and painters. According to his new project, the cathedral was decorated with four columned porticos (in the 1818 project, there were only two—south and north). The central part of the cathedral was emphasized by the sub-dome square formed by four new supporting pylons placed wider than the others. This allowed the main dome to fit clearly into the square of the pylons and prevented its sagging. Four bell towers were installed at the corners of the main volume, seemingly embedded in the walls. They were now closer to the central dome than in the previous project, further strengthening the cathedral's square structure. Its overall appearance became more compact and balanced, with the central dome dominating the composition. The symmetrical porticos allowed the main northern facade, facing the Neva and the Bronze Horseman, to be artistically united with the main western facade, where the temple entrance is located. The northern and southern porticos, with three rows of columns (unlike the single-row western and eastern ones), repeated Montferrand's idea on an enlarged scale of the Roman Pantheon's columned portico, the most famous ancient structure. By order of the Academy of Arts president Olenin, casts of Corinthian capitals from the Pantheon in Rome were shipped to Petersburg, arriving in 1828.

Foundation work began as early as 1818, according to Montferrand's first project. Complex technical issues were to be resolved by Betancourt, but busy with other projects outside Petersburg, he could not address all problems arising during construction. Thus, Montferrand, not an engineer and initially responsible for authorial control over material quality and construction, found himself in a difficult position. Supply and construction organization were not his duties, and he sought to expand his authority and independence from the Commission, which he achieved by 1819.

Construction began with dismantling the existing semicircular apses and preparatory work for new foundation parts. Montferrand emphasized that new foundations should be built up to the granite base level, ensuring even building settlement. For the first year, he requested 506,300 rubles, expecting that enough stone would be quarried during summer to erect the building facades to the entablature level the following year.

Trenches were dug for the new foundation parts, from which water was pumped out. Then, vertically, tarred pine piles 26–28 centimeters in diameter and 6.5 meters long were driven into the ground. The distance between piles exactly matched their diameter. Piles were driven by heavy cast-iron hammers using winches powered by horses. Each pile received ten blows. If the pile did not enter the ground after that, it was cut with the overseer's permission. Then all trenches were connected and filled with water. When the water froze, the piles were cut to a level calculated from the ice surface. According to Montferrand, 12,130 piles were driven under the foundation.

Montferrand used solid masonry for the foundation, believing that "for foundations of large buildings, solid masonry is preferable to any other type, especially... if the building is constructed on flat and marshy soil..." This also allowed better connection of the old Rinaldi foundation with the new one and largely guaranteed the building against dangerous settlement consequences.

Granite monoliths for the cathedral's columns were quarried at the Pyuterlaks quarry near Vyborg. These lands belonged to the landowner von Exparre. The advantages of this quarry site were the large granite reserves, proximity to the Gulf of Finland with a deep fairway, and the postal route. Montferrand noted in his diary upon first visiting the quarry: "The amazement we felt when we saw... the granite cliffs was, of course, great, but it was replaced by genuine admiration when later we admired seven still unprocessed columns in the first quarry..."

The quarry work was initially led by merchant Samson Sukhanov, who also participated in creating the Rostral Columns and Kazan Cathedral. According to a contract from January 1819, Sukhanov acted as the contractor, but by May of the same year, part of the contract for column preparation was transferred to merchant Shikhin. Later, the entire contract was given to Shikhin. In September 1820, the first column for the cathedral was delivered to Petersburg.

The granite monolith extraction method used at Pyuterlaks was described in 1824 by Olenin, who visited the quarry with Montferrand and Stasov. Suitable monoliths without visible defects were located on a vertical granite cliff. Granite layers at Pyuterlaks were separated by earth layers (called rupazes) about half an inch thick. The outline of the blank was marked, then holes were "drilled" across the future column's width with iron drills with pointed ends (lengths ranged from 3/4 to 2 sazhen). "Drilling" involved one worker holding and turning the drill, while two others struck it with heavy sledgehammers until the blank separated along the rupaz. Along the contour marking the column length, holes up to the rupaz depth were drilled, spaced 5–6 vershoks apart. Powder and wads were placed in the holes, and the monolith was separated by powder explosions along the column length. Then, on the "horizontal surface of the cliff," iron wedges were driven in, and workers struck them until the blank separated and fell onto prepared scaffolding at the cliff base. Olenin noted that separating granite masses was difficult and slow work.

The separated granite mass was inspected for defects and size conformity. Then the column blanks were roughly shaped into a "round form" using hammers. Transportation from the quarry was carried out on flat-bottomed vessels specially made at Charles Berd's factory. The column monoliths were rolled down to the seashore, loaded onto barges, and each vessel was towed by two steamers to a pier in Petersburg. There, the monoliths were unloaded and transported along a special rail track to the construction site for final processing. The use of this rail track at the construction site was the first in Russia.

Frequently visiting the quarry, Montferrand noted: "Granite extraction, a kind of work not very common elsewhere, is very often encountered and well understood in Russia... works that excite our admiration for ancient achievements here are nothing but daily routine, which no one finds surprising."

Pile driving, foundations under the four towers, eastern and western porticos, and stylobates of the northern and southern porticos were completed in 1826. The next construction stage was erecting the porticos before building the cathedral walls. This architect's decision, contrary to common practice, was due to the complexity of installing granite columns.

In 1822, Betancourt designed scaffolding and mechanisms for lifting the cathedral's columns, but Montferrand did not use them because construction was suspended. Betancourt's design was used to create a system of mechanisms with which Montferrand installed the Alexander Column on Palace Square in 1832. Scaffolding drawings were made for each portico separately, signed by Montferrand and architects Glinka and Adamini, and approved on June 15, 1828.

Special scaffolding was built for lifting columns, consisting of three tall spans (for large porticos) or one span (for small ones), formed by vertical posts covered with beams. Sixteen cast-iron winches-cabestans were installed on the sides, each operated by eight people. The column was wrapped with felt and mats, tied with ship ropes, and rolled into one of the scaffolding spans; the rope ends were secured to the cabestans through a block system. Workers rotated the winches to bring the monolith to a vertical position. Installing one 17-meter column, 1.8 meters in diameter and weighing 114 tons, took about 40–45 minutes. Montferrand noted in his records that "the wooden scaffolding structure... is so perfect that during all forty-eight column installations, not even a simple creak was heard."

The first column (far right in the northern portico) was installed on March 20, 1828, in the presence of the royal family, foreign guests, many architects who came specifically for the celebration, and ordinary townspeople who filled the square and nearby rooftops. A platinum medal with Alexander I's image was placed under the column's base. The portico construction was completed by autumn 1830.

Then began the construction of supporting pylons and cathedral walls. Granite pads and metal ties of various profiles were embedded in the brickwork on lime mortar for greater strength. Walls and pylons were laid simultaneously around the perimeter. Montferrand's decision to alternate rows of brick and hewn granite in the pylons was innovative, as noted by Rotach and Chekanova, creating an "ideal supporting structure." Granite interlayers with different plan configurations ensured pressure transfer along purely hewn horizontal and vertical surfaces.

The wall thickness ranged from 2.5 to 5 meters. The outer marble cladding was 40–50 centimeters thick, the inner 15–20 centimeters. It was installed simultaneously with the brickwork using iron hooks (nails) inserted into specially drilled holes. Rafters made of forged iron were manufactured for the roof. Ventilation galleries were arranged inside the southern and northern walls. For natural lighting, attic light galleries were made above the galleries.

In 1836, wall and pylon construction was completed, and vault construction began. The built brick vaults were 1.1 to 1.25 meters thick and rested on six pylons. In addition to structural brick vaults, decorative vaults were made, consisting of an iron frame covered with metal mesh and clad with artificial marble. A 30-centimeter space was left between the decorative and structural vaults. Such double vaulting is a characteristic feature of the cathedral, not previously seen in other church buildings in Russia and Western Europe.

In 1837, when the dome base was completed, installation of 24 upper columns began. Columns were raised along an inclined platform—a system of flying buttresses resting on the colonnade base on one side and on walls and pylons on the other. Lifting was done using special carts. For turning the columns, devices made of two cast-iron circles were used, with balls inserted into the groove of the lower one.

The next construction stage was the dome erection. Montferrand aimed to lighten the dome as much as possible without losing strength. He proposed making it entirely metal instead of brick, as planned in the 1825 project. Dome calculations were performed by engineer Lomnovsky. The metal structures were cast at Charles Berd's factory. The dome used 490 tons of iron, 990 tons of cast iron, 49 tons of copper, and 30 tons of bronze. St. Isaac's Cathedral dome became the third dome in the world made using metal structures and shells (after the Nevyansk plant tower in the Urals, built in 1725, and the Mainz Cathedral dome in 1828). The model was the dome of London's St. Paul's Cathedral, designed by Christopher Wren. But Montferrand, borrowing the design, used different materials.

Structurally, the dome consists of three interconnected parts formed by cast-iron ribs: the lower spherical, the middle conical, and the outer parabolic. The metal frame consists of 24 I-beam ribs. The web connecting the I-beam flanges is perforated. Frame parts are bolted together. The outer shell diameter is 25.8 meters, the lower one 22.15 meters. The space between the trusses was insulated with hollow conical pottery pots on brick crossbars, with gaps filled with lime cement mixed with crushed stone. About 100,000 such pots were used for the vaults. Pottery vaults improve the cathedral's acoustics and are much lighter than brick ones, but, as Nikitin notes, they do not protect the dome from freezing.

Thermal insulation of the pottery vaults was made from felt and cow wool with resin and harpius (pine rosin) filling. The felt was covered with a lime-sand mortar painted with oil paint. Open parts of the metal structures were also protected with felt. The inner conical dome was covered with copper sheets painted bluish, with large bronze rays and stars creating a striking night sky effect. The dome's exterior was covered with tightly fitted gilded copper sheets.

The gilding of the cathedral domes from 1838 to 1841 was done by fire gilding. During the process, 60 craftsmen were poisoned by mercury vapors and died. According to contemporaries, from 60 to 120 people (likely including those working on interior gilding details) died from mercury poisoning during the cathedral's construction. Later, gilding of relatively small details was done by a safer electroplating method. In total, 400,000 workers—state and serf peasants—participated in the cathedral's construction. According to documents of that time, about a quarter of them died from diseases or accidents. Total construction costs exceeded 23 million silver rubles. By 1842, the cathedral building was mostly completed, and decoration work began, lasting sixteen years.

The long construction period (40 years) was explained by Montferrand's contemporaries by a prophecy that the architect would die immediately after the cathedral's completion. Indeed, the architect died a month after St. Isaac's consecration. His will—to be buried in the temple—was never fulfilled. The coffin was carried around the cathedral and then handed to his widow, who took his remains to Paris. After Montferrand's death, passersby allegedly saw his ghost wandering the cathedral steps—he did not dare enter the temple. According to another legend, the Romanov dynasty was doomed to fall after the removal of the scaffolding surrounding the cathedral, which remained long after consecration. Coincidence or not, the scaffolding was finally removed in 1916, and in March 1917 Nicholas II abdicated the throne.

During the interior decoration design, Montferrand made three trips to Western Europe to study various church interiors (his first trip was to Italy in 1806 as part of Napoleon's army). In 1842, as Russia's architect, he visited Florence and Rome, carefully studying St. Peter's Basilica in the Vatican. In 1845, he was in Carrara, Genoa, and again in Florence and Rome. In 1851, he was in Carrara, observing marble detail production for St. Isaac's Cathedral. The eclectic thinking of the new historicism era manifested in Montferrand's "Architectural, Artistic, and Historical Description of St. Isaac's Cathedral," in which he attempted to recreate the history of Christian temple architecture and present his project as the most complete, absorbing all previous achievements. However, earlier, the emperor likely doubted Montferrand's ability to create a presentable interior decoration project. When Leo von Klenze visited Petersburg in 1839 to design the New Hermitage, Emperor Nicholas I instructed him to prepare his own interior decoration project for St. Isaac's Cathedral. Klenze criticized Montferrand's project and, in particular, proposed entrusting the encaustic painting technique to German painter Peter von Cornelius, installing stained glass in the altar, and even structural changes to the already built cathedral. A conflict arose. Montferrand sent indignant letters. During his 1842 foreign trip, Montferrand visited London and Paris (to study the interiors of St. Paul's Cathedral and Sainte-Geneviève Church). On the way back, he went to Munich, met Klenze, and convinced him of his project's advantages. Emperor Nicholas I agreed with Montferrand's proposals, and only the altar stained glass from Klenze's project was realized, depicting the resurrected Savior in full height.

Interior decoration work began in 1841, involving famous painters (Fyodor Bruni, Karl Bryullov, Johann Conrad (Konrad) Dorner, Ivan Burukhin, Vasily Shebuyev, Franz Riss) and sculptors (Ivan Vitali, Pyotr Klodt, Nikolai Pimenov). Painting work was supervised by the rector of the Petersburg Academy of Arts, Professor Shebuyev; the decoration project and overall fresco concept were developed by Montferrand, reviewed by the Synod, and approved by the emperor.

One of the main problems was choosing the technique for the painted panels. Initially, Klenze proposed (agreed by Nicholas I) that the cathedral paintings be done in encaustic technique. However, Bruni, involved in discussing the painting method, after consulting with Klenze in early 1842 in Munich, reported that this technique was entirely unsuitable for Petersburg's climate. Relying on restorer Valati's opinion, Bruni advocated oil painting on canvas framed with copper frames with a bottom. Montferrand also leaned toward oil painting. Bruni was tasked with making an encaustic painting sample on copper, but soon it was decided to paint the cathedral walls with oil paints on a special ground, and icons were to be painted in oil on bronze panels.

At the same time, on Montferrand's suggestion, Krivtsov prepared a report on the desirability of establishing a Russian mosaic production, which was not supported by the Academy of Arts. However, on August 12, 1845, Nicholas I "ordered to stop painting icons for St. Isaac's Cathedral on difficult-to-produce copper panels, instructed to paint on canvases, and decreed to 'establish a mosaic workshop to depict mosaics after these icons.'"

According to the work distribution, Bryullov was to paint the main dome (the largest composition of 800 square meters) and the pendentives in the central nave; Bruni—the barrel vault and attic of the main nave; Dorner—12 icons containing 28 images for the side parts of the large iconostasis; Basin—the chapels of Alexander Nevsky and St. Catherine. The western part of the cathedral was assigned to Old Testament themes, the eastern to episodes from Christ's life. Six paintings were made by Italian painter Cesare Mussini.

High humidity inside the cathedral hindered creating a ground resistant to adverse external effects. The wall for painting was plastered, cleaned with pumice, heated with braziers to 100–120 degrees, and several layers of mastic were applied. The low quality of the painting base caused some paintings to be removed and repainted by artists. In some places, the ground detached from the plaster. In his letter dated December 24, 1849, Bruni noted that painting on fresh grounds was impossible due to the subsequent appearance of "saltpeter oxide" on the painting surface. A stable composition was created only in 1855, three years before the painting work was completed.

Because the cathedral's interior conditions—temperature fluctuations, high humidity, and lack of ventilation—were unfavorable for preserving paintings in their original form, from 1851 it was decided (at Emperor Nicholas I's insistence) to use mosaics for interior decoration. Smalt for St. Isaac's Cathedral was produced in the Academy of Arts' mosaic workshop; on December 7, 1864, a special building appeared on the 3rd line of Vasilievsky Island, later called the "Imperial Mosaic Establishment," headed by Bonafede.

Mosaic panel creation continued until the beginning of World War I. Over 12,000 shades of smalt were used in the cathedral's 62 mosaics, with backgrounds made of gold smalt (cantorelli). Mosaic images were made from originals.

Mosaic replaced Zhivago's painting "The Last Supper," paintings on the main dome's pendentives, attic ("Judas' Kiss," "Behold the Man," "Scourging," "Carrying the Cross" by Basin), and pylons. The cathedral's mosaic paintings were exhibited at the 1862 London World's Fair, where they received high praise.

A new tomb was made for the temple, modeled after the tomb of the Church of the Savior on the Hay (author—jeweler Fyodor Verkhovtsev).

On Leo Klenze's proposal, a stained glass window—a Catholic church decorative element—was included in the cathedral interior. The image of the Risen Savior in the main altar window was approved by the Holy Synod and personally by the emperor. The sketch was made by German artist Heinrich Maria von Hess and produced under the supervision of Max Ainmiller, head of the "Glass Painting Establishment" at the Royal Porcelain Manufactory in Munich. The stained glass was installed in the main altar window in 1844 for demonstration to Nicholas I, soon dismantled (according to documents), and reinstalled in 1847.

The monumental-sculptural exterior decoration project (four bas-reliefs on the pediments and sculptures at their corners, attic sculpture and dome balustrades, door bas-reliefs in portico niches) was developed by Montferrand in 1839. The visual program was proposed by Olenin in 1834 and realized with some changes. Remaining entirely within the Empire style scheme, the cathedral's exterior decoration features Empire, Baroque, and Renaissance forms, reflecting a transitional period when classicist principles no longer satisfied artists, and new decorative-plastic principles emerged.

Olenin and sculptor Svintsov proposed involving domestic masters for the exterior sculptural decoration. However, at Nicholas I's request, two pediments (north and east porticos) were executed by French sculptor Lemer. His work was considered not very successful by contemporaries and later researchers. Two bas-reliefs on other porticos were made by Vitali, whose works Montferrand accidentally saw during a trip to Moscow. Portico pediments were cast at Berd's factory from 1840 to 1845. Statues of the 12 apostles (by Vitali) crown the pediments—three of them for the northern portico were made by electroplating at the Duke of Leuchtenberg's factory.

The dome balustrade is adorned with twenty-four angel statues (I. German, 1839–1840), holding various symbols and attributes. Their models were casts from antique statues from the Academy of Arts.

The cathedral's exterior doors (Montferrand's project approved in 1840, influenced by Ghiberti's gates for Florence's Baptistery of San Giovanni) were commissioned in 1845 to Vitali. Made by electroplating at the Duke of Leuchtenberg's factory, the process was supervised by the method's inventor, academician Jacobi.

The total cost of constructing the cathedral from 1818 to full completion in 1864 was 23.26 million silver rubles. Of this, foundation construction cost 2.54 million rubles in assignats; 48 granite portico columns, including installation, cost 2.61 million rubles; wall masonry 2.51 million rubles; marble cladding 7.49 million rubles; construction of four portico pediments 2.28 million rubles; roofing 2.45 million rubles.

The solemn consecration of the cathedral took place on May 30, 1858, on the feast day of St. Isaac of Dalmatia, in the presence of Emperor Alexander II and members of the imperial family. Troops were lined up, whom the emperor greeted before the consecration ceremony led by Metropolitan Gregory (Postnikov) of Novgorod and Saint Petersburg. Tribunes for the people were arranged on Petrovskaya and Isaakievskaya squares; nearby streets and rooftops were crowded with people.

Six years after the consecration and after the construction commission's work ended, in 1864, the cathedral building was transferred to the Ministry of Railways and Public Buildings. Inspector and architect positions were established at the cathedral, and a special "technical-artistic council" of three Academy of Arts professors was created for constant monitoring of the cathedral's condition.

In 1871, St. Isaac's Cathedral was transferred to the Ministry of Internal Affairs.

In 1883, the cathedral's status changed: it (like the Cathedral of Christ the Savior in Moscow) came under dual jurisdiction—under the Orthodox confession department "in economic matters" and remained under the Ministry of Internal Affairs "in technical-artistic matters." At the same time, the question of transferring the cathedral building to sole management by the spiritual department was discussed. Metropolitans of both capitals agreed, but Academy of Arts rector Alexander Rezanov opposed, stating he would not allow a situation where a building costing 15 million rubles and 45 years of labor would be left without specialist supervision and "significant material support from the government." Rezanov's arguments were heeded, and both cathedrals remained under the Ministry of Internal Affairs in "technical-artistic" matters. On May 25, 1883, Emperor Alexander III approved the "opinion of the State Council" on the two cathedrals, recorded as document number 1600 in the third part of the Complete Collection of Laws of the Russian Empire, titled "On the Order of Managing the Cathedral Churches of St. Isaac in St. Petersburg and Christ the Savior in Moscow."

At the beginning of the 20th century, the question of transferring St. Isaac's Cathedral (and Christ the Savior's Church) to exclusive management by the Orthodox confession department arose again. The need was motivated by difficulties in managing the buildings due to dual subordination—to the Ministry of Internal Affairs and the spiritual department. On October 28, 1908, this issue was considered at the Council of Ministers, and the status remained unchanged—dual subordination. Additionally, commissions were formed to determine necessary repairs, including representatives from four ministries—Internal Affairs, Finance, Orthodox Confession Department, and State Control.

Following the decree on separating church and state, the cathedral building and church property were nationalized. In 1918, the building came under the People's Commissariat of Property of the republic, and in December 1919, it was returned for use to the cathedral parishioners. More than 30 people signed the agreement on behalf of the parish, under which the parish used the cathedral free of charge but was obliged to cover current maintenance costs (heating, repairs, security, etc.). In May 1922, during the seizure of church valuables from the cathedral, 48 kg of gold items and over 2 tons of silver decorations were confiscated. On April 29, 1922, its rector, Archpriest Leonid Bogoyavlensky, was arrested. In March 1923, the cathedral came under the management of the parishioners of the Orthodox Russian Church (the Renovationists). The usage agreement with the public organization (parish) was terminated due to improper fulfillment of duties; services ceased when the Presidium of the All-Russian Central Executive Committee decreed on June 18, 1928, to "leave the cathedral building exclusively for Glavnauka as a museum monument." Remaining church property was sold through the State Fund, and all bells were removed and melted down.

On April 12, 1931, one of the first anti-religious museums in Soviet Russia was opened in the cathedral.

During the Great Patriotic War, the cathedral suffered from bombings, artillery shelling, cold, and dampness; traces of shells remain on walls and columns. During the siege, exhibits from museums in Leningrad suburbs, as well as the City History Museum and Peter I's Summer Palace, were stored in the cathedral.

Since 1948, it has functioned as the "St. Isaac's Cathedral" museum. Restoration work was carried out in the 1950s and 1960s. An observation deck was arranged on the dome, offering a panorama of the city's central part. From 1931 to 1986, a Foucault pendulum operated inside the cathedral, visually demonstrating Earth's rotation thanks to its 98-meter thread length. Currently, the pendulum is dismantled and stored in the cathedral.

Church services resumed in 1990 and are now held daily.

Sources:

https://ru.wikipedia.org/wiki/St._Isaac's_Cathedral

https://isaakievskiy.ru/

https://www.culture.ru/materials/205393/10-interesting-facts-about-st-isaac's-cathedral