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Tap water
Tap water is the basic constituent of the fount solution and its characteristics therefore influence the quality of the fount solution. It is therefore vital to properly control the tap water.
If it is not specifically treated, the quality of tap water is variable. Certain irregularities in its quality are responsible for phenomena that disrupt the printing process. From the viewpoint of offset printing, the main characteristic of tap water is its degree of hardness, or TH (Total Hardness), which reflects the hardness of the water.

The TH: the degree of water hardness

The TH of tap water indicates its calcium and magnesium carbonate content. These values vary according to local geological conditions. Water with a low concentration of carbonates is called "soft water" (low TH) and water with a high concentration is called "hard water" (high TH).
In France, the TH of tap water is expressed in French degrees of hardness (°f).
Did you know?
1° French degree (°f) = 0.56° German degrees (°d) = 10 mg/l of CaCO3
Good handling of the fount solution
According to their TH, tap waters are classified as follows:
 In practice, it is not very important to know the TH precisely. The precision of the reactive strip measurement method is sufficient to check the suitability or not of the tap water to prepare the fount solution. 
Did you know?
When there are severe weather disturbances (drought orthunderstorms), the TH can undergo a sudden and significant variation.
 Unsuitable tap water disrupts the printing process.
• Hard water contains salts that block the pores of the rollers and cause them to lose their aptitude to accommodate the ink. Deposits can build up in the pipes and spray nozzles of the fount solution and disrupt the regularity of its operation.
• Soft water promotes emulsification. Moreover, it is considerably more corrosive than hard water, and can lead to rapid oxidation of the plates and corrosion of the mechanical parts of the press.

Treatment of tap water

The fount additive does not eliminate hardness; it can at most only slightly reduce the disadvantages. When the TH reaches 27°f or higher, the tap water should be treated. The principle of reverse osmosis gives excellent results since it enables the TH to be adjusted to 18°f (ideal for the preparation of the fount solution).
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