What's causing the horizontal lines around my slipcast ware?

This is a fault known as ‘wreathing’ and is caused by the slip casting or skinning over before the mould has been filled, leaving boundary lines showing. The reason for this is that the slip is possibly too thin due to an over addition of deflocculent, or the filling rate was too slow. To correct this you need to add more clay and water to lower the concentration of deflocculent, or fill the mould more quickly.