If the glass quickly becomes covered with black deposits after each cleaning, check the fuel and how the fire is managed before changing the cleaning agent. Wet wood, too little air, or logs placed close to the glass can promote soiling. At the same time, the appearance of the deposit alone is not enough to reliably determine a single cause.
First, take a look at the wood
Determine whether the fuel complies with the device's instructions and has been properly seasoned. A dry log on the surface does not tell the whole story about its interior. If you are assessing wood with a meter, check the freshly split surface and interpret the result according to the measurement instructions and the requirements of the stove.
Do not place logs so that they touch the glass. Maintain the layout and amount of fuel specified for the device. Record whether residue appears also after changing the batch of wood; this helps separate the effect of the fuel from other issues.
Check the air supply and the combustion process.
Read the description of air control for your model. Limiting the airflow too early after lighting may change the combustion and promote the deposition of soot. Do not copy the slider settings from another stove, even if its power is similar.
When cold, inspect the available openings and channels that are allowed to be cleaned according to the instructions. If the device has an air supply system at the glass, its operation depends on correct installation and unobstructed passage. Recurring soiling despite proper fuel and maintenance may require checking the device and the chimney draught.
Cleaning removes the effect, and observation helps find the cause.
Vitcas fireplace glass cleaner is used to remove deposits on compatible surfaces. Apply it to cooled glass according to the instructions and do not leave the product on the gaskets. After cleaning, note after which stage of burning the deposit returns and whether it has the same shape.
Do not try to burn off the glass by deliberately overheating the fireplace insert. If soiling is accompanied by smoke in the room or a noticeable change in draught, stop using it and have the cause assessed. A stronger cleaning agent will not fix such a defect.