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Insoluble Salts Can be Removed the Chemical Way Too!

Soluble insoluble salts,removing salt from water,insoluble chloride salts


Calcareous deposits on pottery are easy to remove manually, most of the time. The pottery is wetted first. Then, any tool such as a dental tool, dental burr, scalpel or pneumatic air chisel is used to scrape off the insoluble salts.

Sometimes, a chemical cleaning might be required, especially if shards have to be cleaned. Of course, the general process of a thorough pre-wetting of the pottery (especially the paste) remains the same. If this is not done, the acid that is used for cleansing gets absorbed into the ceramic body. There is a choice of three acids that can be used—oxalic acid, nitric acid, and hydrochloric acid.

The procedure is as follows: wet the pottery item thoroughly, immerse in acid for around and hour or two (to allow for complete evolution of gases), rinse under running water, and dry it.

Now, if you have to make a choice between hydrochloric acid and nitric acid, it is advisable to go in for the former. Since pottery objects come with glazing, the 10% or 20% hydrochloric acid proves safer. Yes, this acid can possibly cause the color of lead glazes to fade, resulting in a milky appearance; but it is still better than nitric acid. Nitric acid tends to be stronger and even a 10% or a 20% solution can lead to dissolution of lead glazes. So unless you are an expert in its use, you better stay away from it.

Oxalic acid is to be used only if iron stains are to be removed. If you should find any, then after completing the above procedure, rinse the item under tap water. Now, soak it in a 10% oxalic acid solution for a few hours. When you feel confident that all the stains have been removed, take out the piece and give it a thorough rinsing under running tap water. Finally, dry it completely.

In the case that you feel apprehensive about keeping your ceramics soaked in acid for such a long time, use a cotton swab soaked in acid. A few drops can be dropped on different locations on the surface each time. You may keep wiping off the acid or rinse the object. The process can be repeated several times. After all, the fear is very real when it is concerning glazed pottery antiques that can never be replaced! Especially, earthenware and terra cotta pieces experience deterioration faster when in contact with acids for lengthy periods of time.

Do you own ceramics that come with a carbonate temper? This would mean objects with a shell or calcium carbonate. Well, if you wish the tempering material in the paste to be retained, hydrochloric acid and nitric acid are both to keep a distance.

On a final note, it is advisable not to play with acids if you are an amateur in the arena of conservation of pottery. It is far safer to go in for mechanical cleaning or just hand over the job to an experienced conservator.

 

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