184 R. T. BAKER AND J. NANGLE. 



presence of impurities within the stone itself. The yellow 

 of many limestones is often due to the presence of finely 

 disseminated iron sulphide. If stone contain either the 

 sulphide or the carbonate of iron, discoloration is a natural 

 consequence of exposure to the atmosphere, and such 

 stone often weathers buff after a few years exposure. The 

 oxides of iron are more stable compounds than the sulphide 

 or carbonate, and are less liable to alteration. 



Discoloration in the face of a marble wall may be due to 

 impurities in the mortar, cement, or brick used in the con- 

 struction, which are brought to the surface through capillary 

 attraction. The iron stains in marble in a wall are prob- 

 ably also due to the ferrous oxide in the mortar, cement or 

 brick used in the construction rather than in the marble 

 itself, and are brought to the surface through capillary 

 attraction. A preventative against ferrous iron in the 

 brick or mortar of the back wall coming to the surface 

 is a coat of asphalt between the back wall and the stone 

 facing, or better still the selection of lime cement, and brick 

 in which it is certain that ferrous iron is not present. 



This question of discoloration of marble requires more 

 attention than perhaps has been bestowed upon it in the 

 past, or otherwise a public prejudice may set in against 

 our beautiful stones. Many of our marbles, such as red 

 and blue Borenore, Kempsey and Springhill, are being 

 extensively used now on shop fronts in Sydney, Newcastle 

 and other towns and it must be admitted with good effect, 

 but will it retain its good name under present treatment ? 

 For instance, is there anything wrong with the polish when 

 the stone does not seem to retain its lustre any length of 

 time ? Is care taken with the backing in order that no 

 foreign substance is likely to cause discoloration by capil- 

 lary attraction? These two defects are beginning to 

 become evident in a few of the marbles now adorning 

 business premises in Sydney, and are only mentioned here 



