Fat washing infuses spirit with the flavor of an oil or fat. Combine spirit with melted fat (bacon, butter, coconut oil, pistachio) at 4:1 spirit-to-fat by volume in a sealed container. Stir to integrate. Hold at room temp 4 hours then freeze 8 hours. The fat solidifies and separates; strain through a coffee filter into a fresh bottle. Examples: bacon-washed Bourbon, sesame oil-washed mezcal. Label, date, store cold. House technique held by senior bartenders only; trainees observe before producing.