Granite is resistant to most cleaning surfactants, short of avoiding aggressive bath and tile cleaners and cleaners that have built in abrasives like comet and soft scrub there are very few restriction. However, it is always recommended to clean natural granite countertops with cleaners specifically formulated for granite. If you decide to use glass cleaners or multipurpose cleaners we recommend that you increase your sealing regiment by 2 times the recommend amount. The reasons are natural stone cleaners are Ph balanced and will not degrade the sealer unlike standard glass and multipurpose cleaners. These cleaners will not harm the granite but will leave the stone vulnerable to staining by prematurely degrading the sealer. For cleaners that are not specifically formulated for natural stone it is recommended to test a small area in an inconspicuous spot to insure there are no adverse affects.
Marble countertops unlike granite have very specific instructions. Marble known for its calcite composition is very sensitive to chemicals. Using cleaners not specifically formulated for marble and other calcite based stones (limestone and onyx) is not recommend. Marble countertops should be cleaned regularly using stone specific cleaners and spills should be wiped up immediately to avoid surface etching and staining. Marble tops should be treated similarly to a fine wood, coasters, placemats, trivets and cutting boards should be used to avoid marring.
A question debated by the masses or at least by those within the stone industry. When speaking of granite, there are some stones that are dense enough too naturally repel foods and beverages. Although, there would be no harm in applying an application of sealer once or twice a year providing it’s done correctly, consider it cheap insurance on a costly item. Then there is the 1-5% of stones that would have an adverse reaction to sealant. These few are so dense that the sealer has nowhere to go and it simply sits on the surface and dries. Any staining or defacement will be done to the sealer and not the stone. Removing the sealer is the answer; however, this usually involves harsh chemicals and should be performed by a professional. Finally there is the other 75%. These stones would all benefit from an application of sealer. Unfortunately, there is no set standard for how often they need to be sealed. This depends on how often you clean, what you clean with and what type of granite is installed. All of this will be discussed at time you select your material when you purchase through Caayu. While all granites may look the same, don’t let anyone tell you they perform the same
Ammonia is not considered to be a effective way to clean natural stone. Generally speaking ammonia is a highly concentrated commercial cleaner that should be used with caution. Due to its high alkalinity it is not recommended for natural stone countertops such as marble and granite as it will certainly speed up the degradation of the sealer as well as affect some of the softer minerals found in certain stones. It is suggestive to use only products that are specifically formulated for natural stone. Such products can be found at your local home improvement and grocery store. Just be certain to check the label and follow the manufactures recommendations.
Direct heat from a hot pot will not damage your granite countertop however; sudden change in extreme temperatures could cause thermal shock resulting in surface cracks. Caayu recommends using hot plates when sudden change in extreme temperature or long term exposure to high temperatures is unavoidable.
Given the information provided it sounds like you have a case of water permeating the surface of the stone countertop. If it is simply water from a clean tooth brush it should not be an issue; if this is the case the water should eventually evaporate within a day or so. If it does, then you need to consider sealing your countertop with an impregnating sealer. Sealers are designed to keep food and beverages from penetrating the surface of the stone. In most cases sealers are simple to use, cost effective and work very well. In some cases, depending on the type of stone used and the amount of use the countertop gets, it’s very difficult to protect it. Also, if the water stain does not evaporate in a few days the stain may need to be drawn out with a poultice.
To answer your question, yes, natural stone will darken over time if not maintained properly. Onyx is a relatively porous material. The darkening comes from permeation. To prevent unwanted absorption from foods and beverages it is necessary to treat the stone with an impregnating sealer. Sealers can be purchased in most big box retailers like Lowe’s and The Home Depot. Depending on where the countertops are installed and how much use they get will determine how often they will need to be sealed. For example a bathroom vanity top made of onyx will need to be sealed as much as three times per year as compared to an end table in a formal living room that might only need to be sealed once every 12-18 months.
Based on the information provided it sounds like your countertops do need to be resealed. Before you seal the countertop it is always best to remove the moisture below the surface. In some cases you may need to stop using the affected area until the water completely evaporates, in my past experience, you could expect it to take a few days to completely dry. To help speed up the process you can pass a hair dryer over the surface, just be careful not to overheat the stone as It could spall. It’s also worth noting that some of the water could be coming from beneath the countertop where the sink meets the underside of the granite. Check to make sure the seal between the two is still good. If not you will need to remove the silicone and redo it.