In the renaissance what caused the demand for slaves to soar?

Crop production and servants

The question - "what caused the slave trade to increase during the early 1800s" - is a little difficult to answer unless one posits that it is a trick question intended to determine whether a particular student has done his or her homework. Having peaked during the mid-18th Century, the slave trade actually began to contract considerably by the end of that century. Debates in Europe and in North America regarding the morality of the slave trade resulted in growing sentiments against the practice, with laws being passed on both sides of the Atlantic outlawing the trade in slaves. Article 1, Section 9 of the U. S. Constitution stated that states could continue to import slaves, but that after 20 years, that right could be abolished:

"The Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight, but a Tax or duty may be imposed on such Importation, not exceeding ten dollars for each Person."

As soon as that 20-year period was over, however, the Congress passed the Act Prohibiting the Importation of Slaves, banning the trans-Atlantic slave trade. That Act's opening provision read as follows:

"Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That from and after the first day of January, one thousand eight hundred and eight, it shall not be lawful to import or bring into the United States or the territories thereof from any foreign kingdom, place, or country, any negro, mulatto, or person of colour, with intent to hold, sell, or dispose of such negro, mulatto, or person of colour, as a slave, or to be held to service or labour."

With the passage of this law, the slave trade was effectively declared illegal. Deep divisions between the northern and southern portions of the country, however, would continue, especially with respect to the issue of slavery. The South's defeat in the Civil War (1860-1865) would finally end the practice once and for all. It is incorrect, however, to suggest that the trade reached its peak during the very period when European colonial powers themselves were increasingly banning the practice. The British, in fact, became militarily active in preventing the trade by dispatching its navy, the strongest in the world, to patrol the coast of West Africa with orders to intercept all vessels transporting slaves.

Beyond issues of morality, another reason for the decline in the slave trade was simple economics. Slaves were an important part of the agricultural economies of many countries, especially in North America, but the onset of the industrial revolution made the manpower requirements that drove the slave trade increasingly obsolete. The American South, of course, was a predominately agrarian society, with plantations providing the bulk of the region's economic wealth. As Europe and the northern regions of the United States ushered in more advanced means of production, the need for slaves diminished.