In 1963, Petts designed and created a stained glass window featuring a Black Jesus for the 16th Street Baptist Church in Birmingham, Alabama, following a racially motivated bombing that killed four African-American girls aged 11–14. Petts was said to be horrified "as a father and as a craftsman" upon hearing the news. Working with the "Western Mail" to raise funds, Petts arranged donations from many thousands of Welsh people to pay for the window. The window was installed and dedicated in 1965. In 1970, the designs for the window were donated to the National Library of Wales in Aberystwyth. In 2013, to mark the fiftieth anniversary of the bombing, Petts's original designs were displayed at the National Library of Wales. In September 2018, it was reported that the church was concerned that Alabama's stormy weather would destroy the window and appealled to the public to raise funds to preserve it.