Gardner-de Horvath film testing apparatus
Gardner-de Horvath Filmometer. Another type of filmometer which gives very concordant results was recently devised by the writer and de Horvath. This apparatus is shown [above].
It consists of a three-necked Wolff bottle having provision at one of its necks for exhausting the air from the bottle. The reverse neck is provided with a gauged glass tube dipping into a porcelain crucible containing mercury, thus acting as a manometer. The middle neck is fitted to accommodate two ground glass plates. Both these plates are provided with a central orifice one millimeter in diameter. Between the plates is placed a small section of paint film. The plates may be pressed together or clamped together and placed over the middle neck of the bottle, a close contact being made with Canada balsam. As the air is exhausted from the bottle, the mercury in the tube will rise and continue in its ascent until the film, which is exposed to atmospheric pressure, has offered it maximum resistance, which is shown by the breaking point. This point is observed on the manometer and the result expressed in centimeters of mercury.
Table of Film Testing Results. By means of the Perry film-testing apparatus, described in the above, interesting results have been obtained, which are embodied in the following table:
Comparative Strengths of Films as Obtained by the Breaking
Machine
| No. Coats | Pressure | Thickness | Stretch | ||
| 1. | Zinc oxide | 3 | 33.2 | 0028 | .30 |
| 2. | Zinc lead | 3 | 32.7 | 0034 | .35 |
| 3. | Asbestine | 3 | 28.0 | 0045 | .15 |
| 4. | Sublimed white lead | 3 | 17.9 | 0024 | .38 |
| 5. | Barytes | 3 | 13.3 | 0042 | .33 |
| 6. | Lithopone | 3 | 13.1 | 0024 | .49 |
| 7. | Whiting | 3 | 13.0 | 0033 | .32 |
| 8. | Quick process white lead | 3 | 11.3 | 0025 | .38 |
| 9. | Gypsum | 3 | 10.8 | 0039 | .29 |
| 10. | China clay | 3 | 10.8 | 0035 | .16 |
| 11. | Silex | 3 | 9.6 | 0032 | .32 |
| 12. | Blanc fixe | 3 | 8.5 | 0030 | .28 |
| 13. | Corroded white lead | 3 | 7.3 | 0020 | .33 |
| 14. | Barium carbonate | 3 | 7.2 | 0028 | .16 |
By means of this machine it is possible to obtain very valuable information concerning the effect of age upon a paint as influencing its strength and elasticity. These are two vital qualities in a paint, as it is through its strength that a paint resists abrasion, cracking, peeling, and blistering. That elasticity is a vital qualification of a paint may easily be seen through the checking of oil paintings, which, as Ostwalt has pointed out, is due to the unequal coefficients of expansion between the ground and the paint. This is particularly noticeable in the alligatoring of many enamels which contain large percentages of zinc.
Curves have been prepared having pressure as an abscissa and elasticity as ordinate. These curves show remarkable differences in different pigments. For instance, in the case of white lead, the curve takes a steep upward trend when it apparently reaches a maximum, the curve then flattening out and finally taking another steep upward trend just before breaking. This may be construed as follows: That under low pressures the white lead film is perfectly elastic, when a maximum is obtained, beyond which elasticity does not extend. This point is the maximum point of the upward trend. From here on pressure may be applied without any increase in stretch, this being represented by the flat part of the curve, while the steep upward trend just before breaking shows where the paint begins to tear, finally culminating in breaking. In the case of asbestine, however, the curve is more of a straight line up to the breaking point, which would go to prove that elasticity is proportionate to pressure in the case of this pigment.
Moisture Absorption. The structure of certain pigments is such that when they are ground in linseed oil and painted out, films are produced which are very water-resistant. This action is possibly due to the filling of the voids in the oil, thus making a compact and water-resistant film. Pigments which are coarse and which present an angular crystalline structure, often produce films which contain a relatively large number of voids and are less waterproof. Certain pigments are chemically active and tend to produce, when ground in oil, metallic soaps which act for a time more or less as varnish gums, in keeping out moisture. Later on, however, such films are apt to break down and admit moisture in quantity. The tests herein described were designed by the author to determine the water-excluding value of a number of typical pigments when ground in linseed oil and painted out into films. Unfortunately, no method has been devised by which films of the same gauge could be prepared. The variations in the thickness of the films used in these experiments, however, are not very great.