Watch your candles closely and learn to read them to help you with your practice.
A strong flame: It means that your energies are raised. There is a lot of power behind your spell and little to no resistance. Indicates that all your power and energy is focused on your manifestation. This is a good sign. If a candle shaped into a figure is used and has a strong flame it means that whoever it represents is either angry, winning, or using authority over another. When their are two figures the candle with the higher flame has authority over the other person.
A weak flame: Indicates that the magick you cast is facing a heavy opposition and is a sign that you may need to recast your spell in order to overcome the opposing force. On a figure candle this may indicate that the representative may be loosing a battle or an argument.
A jumping flame: This indicates strong emotions. There may also be some sort of resistance against you. Maybe close your circle and bring your mind back to focus before continuing. It may also mean an explosion of energy. When two figure candles are being used it often means that an energetic or a heated argument is taking place between them.
A noisy candle: Indicates a conversation. It may may that a spirit is trying to send you a message or get in contact with you. The louder the noises get the more urgent communication must be.
The flame catches something on fire: Someone one may be using malicious magick against you.
A clean even burning candle: Means your spell and manifestations are likely to come true.
Fast Burn: The results of your spell will happen quickly.
Flickering Flame: If there is no breeze then it means a spirit is near by. If it is a devotional flame it means you were able to get in contact with your deity.
Cannot blow the flame out: A spirit may not be finished its conversation with you or you did not complete a spell. The spirit needs to complete its work, something may be may in the works that you do not want to interrupt.
A flame goes out while burning: This means the spirits cannot help you and the answer you seek is already determined.
Black smoke/soot: Your energy and magick is being blocked. Negative energy is being sent your way, whether as a curse or hex. You may soon be faced with tough challenges and a hard road ahead of you.
Smoke wafts towards you: It means your call will likely be answered or your spell has been cast successfully. You will get results quickly.
Smoke wafts away from you: You must work harder in order for your calls to be answered.
Watch the direction the smoke wafts: East – Something powerful is assisting you, North – You are ready to begin your ritual and get results, South – The spell will be successful, West – A powerful interference from an outside force.
Smoking Candle: Negative energy is being burned away.
When a candle leaves wax residue: Your spell may need to be repeated.
Look for symbols in the candle wax, like reading tea leaves.
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May the Moon light you path!
I would add to look for these after a) trimming the wick of any candle used to a short length above the wax, and b) only if the candle is being burned in a room with no drafts, no errant temperature changes, and over the length of the spell being cast. Otherwise, the mundane reasonings for any of these would far outweigh the eldritch.