Add a few drops of low concentration hydrochloric acid, to bring the solution to neutral pH, and to get rid of the traces of piperidine you used as a catalyst (as it will form a salt with the added acid) . It seems you reproduced a literature method, and this was not described or you may have used too much piperidine.

If nothing else helps extract it in an organic solvent of your preferance, dry it, concentrate it to dry under reduced pressure, and try recrystallizing from ethanol, since the procedure you tried seemed to favor this. If it dissolves well in ethanol, make a very concentrated solution, and add a few drops of water.  If it starts to solidify, add a bit more water. Leave it to rest for a little time, and you should be able to filter of a solid.

What does your TLC look like? In what solvent did you try to monitor it? If there are by-products and a main spot the TLC should guide you in the purification of the main compound.

First try to extract products from water with organic solvent (EtOAc, Et2O, etc), than evaporate. If the residue solidifies, try to crystallize it. If not, try to purify you compound by flash or column chromatography. Your situation is common when the desired compound formed during the reaction contains byproducts.

Try to extract your product with hot hexane (if product not go to hexane  add some benzene). More of tar byproducts would NOT dissolve so you can decantate a hot solution form tar products. Refluxing with a small ammount of SiO2 will make the additional purufucation. After cooling would be more chances to obtain solid crystals of product. Of course all this manipulations you have to do if you do not want to use preparative chromatography and decide to make easely. Or if the quantities of product is so big that it impossible to use chromatography in laboratory scale

Your reactants are (a) ketone and (b) substituted benzaldehyde. I understand you are trying cross-aldol condensation. There are many protocol available for carrying out selective cross-aldol condensation in the published literature. 

Aldehydes are, in general, notorious to handle, as they are prone to self-condensation, giving rise to oily products. Preferably, freshly distilled aldehydes should be used. 

Have you tried to keep it under vacuum desiccator overnight? Sometimes small amount of organic solvent may turn the mixture into oil.

My suggestion:

[1 step] Heating under reflux in diethyl ether with the addition of active carbon

[2 step] Evaporate to dryness

[3 step] Crashed under petroleum ether (in ice bath)

Which ketone and aldehyde, their molar ratio? What do you want to receive?

This is a phenomenon called "oiling out".  It arises from the inclusion of small amounts of impurities in the product, preventing crystallization.  Depending on the polarity of the product, recrystallization from refluxing acetone may work to solubilize the remaining starting carbonyl compounds and allow your product to crystallize upon cooling.

Sometime dissolving the oily product in minimum volume of diethyl ether followed by cooling in ice chamber of refrigerator gives solid product. One can take it out and immediately decant off the liquid i.e ether with dissolved gummy mater. The solid residue then can be recrystallized from suitable solvent.

Dal,

You have already received a valuable information from colleagues.  Here, I would like to ask why did you heat your reaction mixture? It is known that such reaction can work smoothly at low to room temp. !  You mentioned that you monitored the reaction by TLC was it a clean one?  byproducts could indeed turn an oily product.  Also, check

--- Dimmrock, J. R. et al Pharmazie, 42(2), 111-13; 1987 

--- Zhang, Min et al Synthetic Communications, 42(19), 2831-2843; 2012

Good luck

All of rhw advice fiven thus ar has merit but one more issue that has not been raised is whether or not your product is actually crystalline. Is it a novel (new) compound or are you making something that someone else has already made? If it is novel there is no way of knowing whether or not it should be crystalline.

Add a few drops of low concentration hydrochloric acid, to bring the solution to neutral pH, and to get rid of the traces of piperidine you used as a catalyst (as it will form a salt with the added acid) . It seems you reproduced a literature method, and this was not described or you may have used too much piperidine.

If nothing else helps extract it in an organic solvent of your preferance, dry it, concentrate it to dry under reduced pressure, and try recrystallizing from ethanol, since the procedure you tried seemed to favor this. If it dissolves well in ethanol, make a very concentrated solution, and add a few drops of water.  If it starts to solidify, add a bit more water. Leave it to rest for a little time, and you should be able to filter of a solid.

What does your TLC look like? In what solvent did you try to monitor it? If there are by-products and a main spot the TLC should guide you in the purification of the main compound.

The product envisiaged could be purified with flash chromatography or solvent extraction and then the spectra taken. It could be that the compound naturally should be an oil. Again you need to find out if the reaction can take place under room temperature so as to avoid heating which could also have caused the problem.

 I ask about the nature of substituted groups on aldehyde and its position? what kind of ketone you use ? you can use KOH or NaOH 50 % to proceeding the reaction instead pipredine at room temp. with out ref. 

I agree with László's answer. You might have added larger quantity of piperidine. Add few drops of dil.HCL to the reaction mixture or quench the reaction into dil. HCl in crushed ice. 

I had faced similar problems in the past. It should definitely work. You could alternatively use inorganic base like 10% NaOH in EtOH and stir at RT overnight. 

I wonder why you need to react for 28h for  Claisen-Schmidt condensation reaction? I think you should monitor the reaction carefully as it might also effect the reaction yield. 

May be that compound nature is oily or If you do column i think you can get solid.

oily mixture from my organic reaction is due to the formation of the mixture of the product or due to incomplete reaction.

Surely this is because of piperidine in excess, usually a Mannich base formation takes place in the first step then enolate of ketone will react to this Mannich base to give u the desired product. At the end of the reaction the catalyst which is piperidine kicked outwhich will make the mixture oily. So now u need to remove it. With due respect i will suggest vacuum distillation at 120 C for 2 -3 hrs inorder to remove Piperidine.

You should check the NMR to see if there is some unreacted aldehyde or ketone. Then you should try to obtain pure product with column chromatography. Try avoiding keeping your compound in open atmosphere. seal it well and keep in refrigerator.