A mint operated in Kluczbork during the reign of Duke Bolesław III the Generous, in the early 14th century. In 1426 Duke Louis II of Brzeg granted Kluczbork privileges of a salt market. For centuries the town was inhabited by a predominantly Polish-speaking populace. The textile industry began to grow in importance in 1553, but suffered a fire in 1569. Another great fire destroyed many houses on December 8, 1562. On January 25, 1588, the day after the Battle of Byczyna, Polish troops under Jan Zamoyski plundered the city. The townspeople accepted the Protestant Reformation in 1656 and converted the local Roman Catholic Church into a Lutheran one. The Polish Brethren settled in the city after 1660, and organized their synods in the city in 1663 and 1668. The town had a population of approximately 1,000 in 1681.