The high demand for slaves was due to a shortage of laborers in Portugal. Black slaves were in higher demand than Moorish slaves because they were much easier to convert to Christianity and less likely to escape. Although it was more expensive to purchase a slave than it was to employ a freeman, the sparse population and the lack of free labor made the purchase of a slave a necessary investment. The number of black slaves in Portugal given by contemporary accounts argue that Lisbon and the colonies of Portugal averaged a maximum of 10% of the population between the 16th and 18th centuries, but these numbers are impossible to verify. Most slaves in Portugal were concentrated in Lisbon and to the south in the Algarve. The number of black slaves brought to Lisbon and sold cannot be known. This is because the records of both royal institutions responsible for the sale of black slaves, the Casa de Guiné and the Casa dos Escravos were damaged during the earthquake of 1755 in Lisbon, and the fiscal records containing the numbers and sales of these companies were destroyed. The records of the royal chronicler Zurara claim that 927 African slaves were brought to Portugal between 1441 and 1448, and an estimated 1000 black slaves arrived in Portugal each year afterward. A common estimate is that around 2000 black slaves arrive in Lisbon annually after 1490.