Question: Trade beads (sometimes called aggry and slave beads) were otherwise decorative glass beads used between the 16th and 20th century as a token money to exchange for goods, services and slaves (hence the name), this included the African slave trade, slavery has existed in Africa for how long?
Context: Slavery in Angola existed since the late 15th century when Portugal established contacts with the peoples living in what is the Northwest of the present country, and founded several trade posts on the coast.  A number of those peoples, like the Imbangala and the Mbundu, were active slave traders for centuries (see African slave trade).  In the late 16th century, Kingdom of Portugal's explorers founded the fortified settlement of Luanda, and later on minor trade posts and forts on the Kwanza River as well as on the Atlantic coast southwards until Benguela.  The main component of their trading activities consisted in a heavy involvement in the Atlantic slave trade.  Slave trafficking was abolished in 1836 by the Portuguese authorities. The São José Paquete Africa (also, "São José-Paquete de Africa") was a slave ship from the Kingdom of Portugal that sank in 1794 off the coast of Cape Town, South Africa.  Close to shore, but in deep water, 212 of the 400 to 500 African slaves who were aboard died when the ship sank.  In 2015, the Smithsonian's African American History Museum, South Africa's Iziko Museums, the Slave Wrecks Project, and other partners, confirmed discovery of the wreck near where it sank.  The ship and its slaves were headed from Portuguese Mozambique to Colonial Brazil, during the height of the international African slave trade.  Few other former slave ships have been found, but the "São José" is the first and only shipwreck discovered, as of June 3, 2015, of a working slave ship, which sank in transit with its human cargo aboard. The Atlantic slave trade or transatlantic slave trade involved the transportation by slave traders of enslaved African people, mainly from Africa to the Americas, and then their sale there.  The slave trade used mainly the triangular trade route and its Middle Passage, and existed from the 16th to the 19th centuries.  The vast majority of those who were enslaved and transported in the transatlantic slave trade were Africans from central and western Africa, who had been sold by other West Africans to Western European slave traders (with a small number being captured directly by the slave traders in coastal raids), who brought them to the Americas.  The South Atlantic and Caribbean economies especially were dependent on the supply of secure labour for the production of commodity crops, making goods and clothing to sell in Europe.  This was crucial to those western European countries which, in the late 17th and 18th centuries, were vying with each other to create overseas empires. Trade beads (sometimes called aggry and slave beads) were otherwise decorative glass beads used between the 16th and 20th century as a token money to exchange for goods, services and slaves (hence the name).  Trade beads were used to exploit African resources by early Europeans.  This included African slave trade.  The beads were integrated in Native American jewelry using various beadwork techniques.  Aggry beads are a particular type of decorated glass bead from Ghana. The Barbary slave trade refers to the slave markets that flourished on the Barbary Coast of North Africa, which included the Ottoman provinces of Algeria, Tunisia and Tripolitania and the independent sultanate of Morocco, between the 16th and middle of the 18th century.  The Ottoman provinces in North Africa were nominally under Ottoman suzerainty, but in reality they were mostly autonomous.  The North African slave markets were part of the Arab slave trade. Slavery in Cuba was associated with the sugar cane plantations and existed on the territory of the island of Cuba from the 16th century until it was abolished by royal decree on October 7, 1886.  More than a million African slaves were brought to Cuba as part of the Atlantic slave trade; Cuba did not end its participation in the slave trade until 1867.  As the slaves outnumbered the European Cubans, a large proportion of Cubans are descended from these African slaves, perhaps as many as 65% of the population. Abolitionism is a movement to end slavery, whether formal or informal.  In Western Europe and the Americas, abolitionism is a historical movement to end the African and Indian slave trade and set slaves free.  King Charles I of Spain, usually known as Emperor Charles V, following the example of Louis X of France who abolished slavery within the Kingdom of France in 1315, passed a law which would have abolished colonial slavery in 1542, although this law was not passed in the largest colonial states, and so was not enforced.  In the late 17th century, the Roman Catholic Church, taking up a plea by Lourenco da Silva de Mendouca, officially condemned the slave trade, which was affirmed vehemently by Pope Gregory XVI in 1839.  An abolitionist movement only started in the late 18th century, however, when English and American Quakers began to question the morality of slavery.  James Oglethorpe was among the first to articulate the Enlightenment case against slavery, banning it in the Province of Georgia on humanitarian grounds, arguing against it in Parliament, and eventually encouraging his friends Granville Sharp and Hannah More to vigorously pursue the cause.  Soon after his death in 1785, they joined with William Wilberforce and others in forming the Clapham Sect. Anglican prayer beads, also known as the Anglican rosary or Anglican chaplet, are a loop of strung beads used chiefly by Anglicans in the Anglican Communion, as well as by communicants in the Anglican Continuum, and by members of the Anglican Ordinariates within the Roman Catholic Church.  Anglican prayer beads were developed in the latter part of the 20th century within the Episcopal Diocese of Texas and this Anglican devotion has spread to other Christian denominations, including Methodists and the Reformed; as such they also called Protestant prayer beads. Slavery has existed in Africa for many centuries, and still continues in the current day in some countries. The Irish slaves myth is a conflation of the penal transportation and indentured servitude of Irish people during the 17th and 18th centuries on one hand, and the chattel slavery of Africans kidnapped for the Atlantic slave trade and their descendants on the other, usually used to undermine contemporary African American demands for equality and reparations.  It is also employed by some Irish nationalists, both to highlight historical British oppression of Irish people and to obscure the fact that some Irish people benefited from the African slave trade.
Answer: many centuries