Costs and prices of the slate trade

There were various phases of transformation before the extracted material could be positioned onto buildings, a whole process which generated additional costs and thus added to the price of construction. The pieces had to be slit open and cut into “normal” dimensions. Data from the early 17th century mention “large” and “small” pieces of slate, but the precise measures remain unknown. More information is available for the 1670s, mainly on the weight of the slate pieces sent to the palace of El Escorial to repair the roofing, which had been destroyed in the fire of 1671. From those documents we learn that the slate pieces weighed in average slightly more than a kilogramme per unit.

Documents from the mid-18th century indicate that a standard piece was half a vara long and about a jeme wide. This means that it measured approximately 42 cm in length (1 vara is 836 mm), and 15-18 cm in width, and is thought to have been half a finger thick (about 0.5 cm). Depending on the dexterity of the slitter-cutter and the natural characteristics of the material, the piece was more or less homogenous. In addition, the final calculation of costs had to include the transportation from the quarry to the construction site, a crucial aspect since it was a very heavy material, which meant a considerable proportional increase in price the further it had to be carried from its place of origin. The weight of the pieces − which, as stated above, exceeded 1 kg per unit − was thus an important factor.

Once cut into the correct shape, the pieces of slate were tied together with ropes made of esparto grass and stacked into loads and bundles. The latter were loaded onto mules, donkeys or carts. If carts were to be used, the pieces were placed on loading platforms near to which the carts could be brought in order to receive the load more easily. The pieces, however, were vulnerable to the unavoidable clattering during transportation, which is why mules were preferred for this purpose. They were also faster. A journey to Madrid with ox carts could last ten days, whereas mules only took four. In 1674, the overseers of the repair of the roofing in El Escorial recognised that:

“although transportation by cart entails a lower initial price, experience has shown that it ends up being less costly to bring the slate on mounts at the price indicated earlier rather than on carts at a lower price, since the saying ′Buy cheap, pay dear′ is nowhere truer than in this instance. Indeed, the carts have to traverse the mountain pass of Guardarrama and the shocks crack the slate, much of which then reaches destination shattered, but must be paid as if it had arrived in perfect condition. Even with the pieces which are in a good state upon arrival, much loss has been known to occur, as many of them break open when the slate masters try to place them because they have been damaged by the clatter on the road…”

The contracts recognised the issue of the fragility of the pieces, as can be seen in the shipping instructions to Uclés in 1604, which indicate that the transporters would not be penalised if no more than thirty pieces out of a thousand arrived shattered, that is, a 3% margin. For the construction of the convent of La Visitación in Madrid in the mid-18th century, most transportation was done on animal back and carts were used in less than 25% of cases. Among the 250.000 slate pieces received in 1755, only 1.3% were unusable.

In the early phases of the royal works in the 16th century, the cost of slate was calculated by adding up all the salaries and costs related to the exploitation − workers’ wages, materials and tools, supervision costs − as well as the transportation costs. In fact, the order of slate for the construction of the house of Diego Vargas was accepted under the condition that the final bill should include all these costs and be charged to the King’s secretary. However, during the reign of Philip II, the price of 1 cuartillo (8.5 maravedis) was established for each piece of slate, which meant that the initial cost of ordering 1000 slate pieces was 250 reales, to which transportation costs had to be added. In the early 17th century, contracts were drawn for slate at 9 maravedis per piece, a slightly higher price than before. The cuartillo price was in force in 1673, when the roofing of El Escorial was reconstructed, as can be seen in the bills and the delivery notes drawn up at the time.

However, a report from 1711 warns that, in the years following the repair of the convent, the excess of supply had led to a sharp drop in prices down to 140 reales for 1000 slate pieces − the price indicated in the contracts for the roofing of the royal stables in El Escorial − but that the price moderately increased again to 160 reales in the early 18th century. An order from 1711 shows the price to be at 122 reales for 1000 pieces, but the quarry administrators claimed that they could not extract for less than 150 reales,

“given the high cost and the amount of work required in comparison to the benefits for the quarry officials and workers”

In any case, slate was an expensive material to cover roofs. Looking at the repair of the roof in El Escorial between 1671 and 1674, for which more than 400.000 slate pieces were used, the price of each piece was 8.5 maravedis in the quarry, but, adding the transportation costs, the final price paid at the construction site was slightly more than half a real. That is practically the same price as the one in force a century before during the construction of the monastery. Comparing with the cost of brick, it must be noted that the contracts for El Escorial show the price of brick tiles to be at 25 reales for 1000, that is, less than 1 maravedi per piece.

Nonetheless, the high price of slate was also an indicator of status in an extremely hierarchical society. It is thus unsurprising that slate was seen as a reflection of the preeminence of those who could afford to cover their mansions and buildings with this material: the Crown, the nobility and the Church, as well as the urban wealthy elites. This also explains why the majority of requests for slate were for the upper parts of buildings, such as church towers or spires, and why, in royal works in the 17th century − such as those for El Buen Retiro, a palace which was the true image of the monarchy − only slate was used for the spires, although the rest of the construction landscape was dominated by cheap materials.

⁽²⁰⁾ Los cargaderos se reseñan en las contratas que en 1627 se realizan para llevar material a la Fresneda.

⁽²¹⁾ AHN, Consejos, leg. 17804, año 1674.

⁽²²⁾ AGP, Cª 13541.

⁽²³⁾ Ver por ejemplo la contrata con Juan García Barruelos para el cuarto de la Fresneda en 1627 AHPS, Prot. 7658, fs 182 y ss.

⁽²⁴⁾ AHN, Consejos, legs 17804 y 17805. Un real tiene 34 maravedís y un cuartillo suponen 8,5 maravedís.

⁽²⁵⁾ AGP, Patrimonio S. Ildefonso. Cª 13541.