Samples of slags from the site of Henry's Cort's original iron puddling furnace at Fontley Forge, Hampshire, and a puddled steel works at Roxbury, Connecticut, have very similar compositions that are in agreement with what is known of the operation of puddling furnaces. Fayalite is the dominant constituent and is accompanied by varying proportions of magnetite, silica in the form of tridymite or cristobalite, iron sulphide, and a groundmass composed of complex intergrowths of magnetite, fayalite and glass. Wüstite and metallic iron are absent and the content of Na₂O, K₂O, CaO, and Al₂O₃ is very low. These characteristics distinguish the slags studied from bloomery and finery slags and, if found in other examples, may be diagnostic of puddling slag. In the three principal methods of making wrought iron -- bloom smelting, fining, and puddling -- solid iron is formed in a liquid slag that approximates the composition of fayalite (2FeO SiO₂). Chemical analyses of the bulk compositions of slags from the three processes overlap to such a degree that a given slag sample cannot be associated with the process it is from on the basis of composition alone. Additional information about a slag can be obtained from its microstructure. Compilations of slag microstructures have been published by Sperl and by Bachmann while Kresten has presented a detailed study of the chemistry and mineralogy of ancient Swedish slags. Bloomery slags have received the most attention in recent research. Fells has described the structures of several English bloomery slags in detail and has also reviewed the results of previous research on slag structure. Micrographs of finery slags have been published by Bedwin and by Morton and Wingrove; inspection of their photographs shows that there are no differences in microstructure that could be used to distinguish finery from bloomery slag. No examples of the microstructure of puddling slags appear to have been published. We have examined the structures of samples of puddling slags from Fontley Forge (Hampshire, England) and Roxbury (Connecticut, United States) to see if their structures distinguish them from finery and bloomery slags. -- AATA