Wei  T’o  pH Spray measures the acidity of paper over the pH range of 1 to 11.  The pH value is obtained by wetting the paper and matching its color after treatment to the shade of the colored chip on the pH spray can.  In making this comparison, the original color of the paper being tested should be mentally subtracted from its color after treatment.

The purposes of Wei T’o pH Spray are to demonstrate the benefit of deacidifying paper and to prove that uniform deacidification was achieved. To demonstrate this benefit, cover half of a waste piece of acidic paper and deacidify the exposed portion with Wei T’o Spray.  (No. 10 is convenient because it dries rapidly).   Remove the cover and let the sheet dry.  Then spray half of the untreated portion and half of the deacidified portion.  Compare the different colors with the colored chips on the pH spray can to demonstrate deacidification has changed the paper from an unstable, acidic condition to a stable alkaline condition.

To prove the uniformity of spraying, deacidify a dummy sheet.  After it dries, spray it again with pH spray.  The test sheet should have a uniform color, between a pH of 8 to 9, to demonstrate that deacidification occurred on both its front and backsides.  Several explanations are possible if the color varies: (1) the application of spray was uneven; (2) insufficient deacidification spray was applied to completely neutralize all of the acid; (3) air pollution and aging produced very different pH values in various portions of the paper; or (4) spraying is required on both sides because the paper is extra thick.

Most conservators believe the value of very acidic pH paper never falls below about 3.2 to 3.5 because, using the traditional pH electrode test methods, lower values are not produced.  However, light applications of pH spray do show surface pH values of 1-2, the lowest values Wei T’o pH spray will measure.  pH values in this range are predictable on the surfaces of paper in contact with oxygen and acidic air pollutants.  These gases react with paper components to form organic acids whose buffered pH values are around a pH of 1.  The reason these pH values are not measured by the standard contact or cold extraction pH aqueous techniques is these methods produce averages; they do not measure the pH value of a single paper fiber, but rather the average pH throughout a much larger area of the paper of test samples.  Moreover, the pH solutions being measured are diluted directly in relation to the quantity of water used.  By comparison, a light application of pH spray is believed to measure the pH value directly on the surface, i.e., the paper fibers onto which the image the paper is carrying is attached.

This pH variation suggests conservators may wish to reconsider the necessity of deacidifying unbuffered, near neutral paper more seriously than has been done to date.  Papers in the 6.5 to 7.5 pH range could have surface pH values that suggest acid attack could put the long-term stability of images in serious jeopardy.

NOTE:  (1) Caution must be exercised when using Wei T’o pH spray.  The stain is permanent and difficult, if not impossible, to remove without damage.


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