# Study About the Influence of Diatoms on the Durability of Monumental Limestone
## 2 Materials and Methods
### Methodology
#### 2.2.5 Water Absorption Test by Pipette Method  
The UNE-EN 16302:2016 [34] standard defines the procedure to measure water absorption using the Karsten tube pipette method, which determines the amount of water absorbed by porous inorganic materials commonly found in cultural heritage. These materials include natural stones, such as limestone and marble, as well as artificial materials such as bricks, mortar, and plaster.  
The test aims to measure the degree of water penetration under a pressure similar to that of rainwater. Its main advantages are that it is nondestructive, can be performed both in the laboratory and in situ, and is applicable to untreated materials as well as those that have been treated or subjected to aging processes.  
Water absorption is quantified by measuring the volume of water, in ml, transferred from the pipette to the material through a defined test area, expressed in cm\({}^{2}\). This area is determined by the diameter (D) of the contact surface between the Karsten tube and the sample, which, in this case, is 3 cm.  
Results are expressed as ml/cm\({}^{2}\), indicating the amount of water absorbed by the material's surface after a fixed time, measured with a stopwatch accurate to 1 s. A sealing material is applied to ensure proper fixation of the Karsten tube to the sample surface. This sealant must prevent water leakage during the test and must not damage or alter the sample surface. For the present work, butyl rubber was used for sealing. Although the test is considered nondestructive, removing the tube may cause slight detachment of surface material, which could affect the behavior of diatom-treated samples in subsequent tests.  
Therefore, samples tested with the Karsten tube method were not reused for further analyses. To avoid water leakage at the interface between the sample surface and the Karsten tube, the surface must be uniform, flat, and free of visible cracks. The area tested must be larger than the diameter of the test area (D).  
Depending on the material's heterogeneity, the number and size of test surfaces may vary, but at least three samples or different surfaces must be evaluated per series. For this experiment, six samples were tested: three with diatoms (D1, D2, and D3) and three as control samples (C1, C2, and C3).  
Prior to testing, samples were dried until a constant mass was achieved, using a ventilated oven at 68 \({}^{\circ}\)C for 4 days. Before attaching the Karsten tube and starting the test, samples were stored in a desiccator until reaching room temperature (23 \(\pm\) 1 \({}^{\circ}\)C). Once cooled, the Karsten tube was fixed to the surface with light pressure, carefully filled with distilled water to the 0 level on the pipette scale, and the timer was started. The procedure was performed carefully to ensure no air bubbles were present within the pipette and no leakage at the connection.  
The standard recommends recording the water level change at intervals ranging from 10 s to 1 min initially, and then every 5 min until a steady value is reached, with the test ending after 1 h if equilibrium is not achieved. However, during the experiment, no significant changes in water level were observed at 5 or 10 min intervals for most samples. Consequently, a new criterion was adopted: recording the time taken for each sample to absorb increments of 0.1 mL of water, corresponding to the smallest division on the pipette scale. Additionally, water absorption after 24 h was also recorded.  
Figure 9 illustrates the full water absorption test setup, including the Karsten tube fixed to both control and diatom-treated samples.