The muffles are made of refractory clay.
"Young's Demonstrative Translation of Scientific Secrets"
Daniel Young
And so is Death-who muffles up his harsh features and speeds the heavy plough with a step like that of Hope.
"Holbein"
Beatrice Fortescue
Your wife is to inherit the property of a maternal uncle, a gouty old fellow whom she humors, nurses, caresses, and muffles up; to say nothing of her father's fortune.
"Petty Troubles of Married Life, Part First"
Honore de Balzac